In this edition: Seven things to watch tonight, the impeachment risk for Democrats who supported it, and explaining what the heck is going on with the Affordable Care Act.

I filed from a mall today, as is the Los Angeles custom, and this is The Trailer.

LOS ANGELES – No other Democratic debate had such high barriers to entry. No other debate came so close to collapsing, with a labor dispute at Loyola Marymount University resolved just 55 hours before the candidates walk onstage: former vice president Joe Biden; Sen. Bernie Sanders (I-Vt.); Sen. Elizabeth Warren (D-Mass.); South Bend, Ind., Mayor Pete Buttigieg; Sen. Amy Klobuchar (D-Minn.); investor Tom Steyer; and businessman Andrew Yang.

Follow The Post's debate coverage here.

And no other debate will have faced such a struggle to break through. Today's three-hour, seven-candidate contest, hosted by PBS NewsHour and Politico, will unfold right after an impeachment vote that could change the race's trajectory. It won't include Mike Bloomberg, who in one month has already spent more than any other Democrat on campaign advertising. It's been sniped at for weeks because four of the nonwhite candidates still in the race did not get the poll numbers needed to qualify.

Yet, with all that, this is set to be the longest sustained grilling of the primary, with the most time for individual candidates to explain themselves or argue with each other. Here's what could come up.

The impeachment sacrifice. Three of the Democrats on the stage tonight – Sanders, Warren and Klobuchar – are likely to spend some part of January in an impeachment trial. That has loomed over their campaigns for weeks and given each of them a topic that allows them to demonstrate total selflessness.

"I took an oath of office, as did everyone in Congress and part of that oath of office is the basic principle that no one is above the law," Warren said last month in New Hampshire. "If the House goes forward and sends an impeachment over to the Senate, then I will be there for the trial.”

"I guess I’m just going to have to try to be in Washington, D.C., in Des Moines, Iowa, and Concord, N.H., at the same time," Sanders joked to ABC News last week.

"I meet whatever obstacle is put in front of me," Klobuchar said this month on CNN. "This is more than an obstacle. It's my constitutional obligation."

Impeachment has been one of the single most unifying issues in these debates, giving each candidate 60 seconds to deliver the anti-Trump arguments that their voters want to hear. It's a layup for the senators in the race and a test for Biden, whose strength in rounds about impeachment has been shaped by whether moderators ask about Republican attacks on his son Hunter's work for a Ukrainian company. 

If the subject is Trump and electability, Biden does well; if it's about the details of the Hunter Biden saga, he can struggle. But his fellow Democrats have never shown much interest in exploiting that. 

Medicare-for-All Part VI: Medicare's Revenge. Strange, but true: Warren's campaign had wanted more questions about health care at the last debate. In November, Warren released a series of tax increases that could pay for Medicare-for-all, then a plan to transition over to a single-payer health-care system. 

Both plans differed from the approach of Sanders. Both plans had support from Medicare-for-all advocates, though some had already endorsed Sanders. The short version: Warren would implement much of Medicare-for-all before scheduling a vote to ban duplicative insurance, while Sanders would try to pass the entire legislation, with the benefits and the ban, in his first year.

It's a thorny argument but not too hard to explain, and Warren has been explaining it succinctly at campaign stops. Most voters haven't heard it. As recently as this weekend, Warren was still getting questions about how her plan differs from Buttigieg's "Medicare for all who want it," as voters and reporters remain more fixated on why she altered her position than what the position is. (Warren's plan would create a universal system with no deductible, and Buttigieg's would not.)

Warren and her allies have been unable to exploit Buttigieg's position, that a slower transition that leaves private insurance untouched would get to Medicare-for-all, eventually. She now focuses on how fast presidential action in 2021 would get that process started more quickly. How will the candidates who support a public option (Biden, Buttigieg and Klobuchar) rebut that? Buttigieg has recently talked more about how single-payer would disrupt middle-class insurance jobs; Biden and Klobuchar tend to focus on the plan's steep path through Congress. 

All of them have the option of trying to divide Warren from Sanders, which did not happen in previous debates. Even as some of Sanders's surrogates have attacked Warren's revised plan, he has largely refused to. Both Sanders and Warren are far more comfortable attacking Buttigieg, whose work on health insurance at McKinsey & Co. was not known until after the last debate and who succumbed to pressure to release the names of donors, which include some drug company executives.

How to buy an election. The Democratic primary started with a truce on campaign spending – no super PACs, no self-funding. That deal fell apart in the past few months, as Biden gave the nod to a super PAC and Bloomberg leaped into the race with an eight-figure TV buy. And it's hardly come up as an issue in the debates. Biden's super PAC decision, for example, led to exactly zero questions when Democrats met last month.

Warren and Sanders want that to change. They have been more aggressive recently in attacking Bloomberg's largesse and the influence of big donors in the Buttigieg and Biden campaigns. 

"Why would many, many billionaires be contributing to candidates if they didn't think they were getting something out of it?" Sanders asked, in a new CBS News interview.

"We know that one Democratic candidate walked into a room of wealthy donors this year to promise that 'nothing would fundamentally change' if he’s president," Warren said last week in New Hampshire. "We know that another calls the people who raise a quarter million dollars for him his 'National Investors Circle,' and he offers them regular phone calls and special access." She was talking about Biden, then Buttigieg.

Buttigieg and Warren have been at this for weeks, with him channeling the frustrations of a lot of Warren critics: She came up with her no-fundraisers policy only after she began running for president. "The thing about these purity tests is the people issuing them can’t even meet them," Buttigieg told The Washington Post's Robert Costa last week.

Everybody loves Bernie. You can't say Bernie Sanders has gotten off easy in this primary. He's dismissed as unelectable on cable news panels; analysis of polls where he's done well has sometimes ignored him completely; his heart attack, in October, was covered as the potential end for his campaign.

And yet, for strategic reasons, Sanders has rarely been attacked onstage and never attacked by name in paid media. Even when the attacks have come, they've tended to lump him and Warren together as the candidates who cause "electability" night sweats. ("Both Senators Warren and Sanders want to replace Obamacare with Medicare-for-all," moderator George Stephanopoulos said in the September debate, giving Biden a cue to defend the 44th president's legacy.)

Sanders's improvement in recent polls hasn't changed the way most rivals think of him: that he has a high floor of support but a low ceiling, pushed by frustrations about his long 2016 primary bid, his age and his health. They previously praised him for honesty as a way of criticizing Warren on health care; they could now accentuate how he has no billionaire donors (Warren has a couple) or how he's never had corporate clients (she did, as legal adviser). That's left Sanders with some of the highest favorable ratings in the field. Will he benefit, once again, from candidates turning their fire elsewhere?

Does everybody still hate Pete? In November, the Indiana mayor got off relatively easy: Rep. Tulsi Gabbard (D-Hawaii) tangled with him on war powers, and Klobuchar got a chance to repeat her rhetorical question of why a man with a short résumé gets to be a top presidential candidate. But most of the candidates' weapons went unused, from Buttigieg's bungled roll-out of a "Douglass Plan" for black America to the aforementioned support from big donors.

Buttigieg was the first Democrat to benefit from a negative attack in these debates, after he ribbed Warren for being initially unable to say whether she'd raise taxes to pay for Medicare-for-all. Warren has kept her high favorable ratings with Democrats, in part, by swatting away attempts to get her to argue with other candidates. The candidate to watch: Klobuchar, whose path to relevance in Iowa depends on pulling voters away from both Buttigieg and Warren.

Is that all there is? Congress may soon approve the USMCA, an update to NAFTA that both Democrats and the Trump administration call a new standard for trade negotiations. A National Defense Authorization Act just passed, with a go-ahead for a "Space Force" but no restraints on presidential war power. Neither topic has gotten much attention in a debate this year. Both could show the real differences between these candidates, and how they would or wouldn't tack away from positions that hurt the party in 2016. 

The question is whether moderators go there, on topics that haven't been sparking debates on the trail. Sanders, in particular, has been comfortable pointing out a Biden voting record on war, taxes and trade that directly clashes with his own. In these debates, typically, he makes a few criticisms and they're never followed up.

The other two. Andrew Yang and Tom Steyer just barely qualified for the debates, with mixed-to-negative reactions from Democrats. The two outsiders have little in common: Steyer's a billionaire, while Yang has one of the most cash-poor campaigns. But both will get more time to speak than in any previous debate, and both have grown comfortable attacking the better-résuméd candidates as politicians more concerned with ideology than on fixing the political system.

"None of them has a private-sector track record of creating jobs," Steyer has said of the leading Democratic candidates.

Yang, who has broadened his campaign beyond his initial pitch for a universal basic income, is also getting a bit more comfortable in criticizing his rivals, pointing out his crossover support from Republicans. But neither is playing by the same rules as the five candidates with elected experience.

READING LIST

"Impeachment split screen: As House votes in Washington, Trump rallies in Michigan," by Josh Dawsey and Ashley Parker

Seeking solace from the crowds on one of his presidency's worst days.

"How Harvard made Pete Buttigieg the moderate that progressives love to hate," by Michael Kruse

The making of the Democratic leadership class, as seen through one candidate.

"Elizabeth Warren to hold private meeting with tribal leaders Sunday in effort to atone for past claims," by Annie Linskey

Dealing with the issue that nearly ended her career as she arrives in Oklahoma.

"The impeachment trial could keep senators in D.C. at the height of the campaign. How Warren, Sanders and others plan to survive," by Brittany Shepherd and Andrew Romano

The scheduling magic, surrogate work and other tactics for Democrats who will get grounded next month.

"‘It’s depressing, isn’t it?’: With little protest, GOP succumbs to Trump on spending," by Robert Costa

Another big spending vote, and another burial for fiscal conservatism.

"Inside Biden's brain trust," by Ryan Lizza

The team currently winning the Democratic primary is less left-wing, and often more white, than the teams around top 2020 rivals.

IMPEACHMENT WATCH

The latest on the impeachment of President Trump

AD WATCH

American Action Network, "Ignored Us." Most of the anti-impeachment ads voters will have seen by Christmas came from this pro-Republican group, which has personalized its spots in districts won by the president in 2016. The message is the same throughout: focusing on "a politically motivated charade" that made Congress "ignore the issues we care about."

Cory Booker, "Together." Booker has run radio ads before, but never a TV ad, which makes this national spot his debut in a space owned by Tom Steyer and Mike Bloomberg. "I'm not a billionaire," Booker says, by way of explaining why he's not on the debate stage. "It's about all of us, standing together, fighting together: Not just to beat Donald Trump, but to bring about the transformative change we need."

Julián Castro, "Truth." Like Booker, Castro is elbowing into the Democrats' debate conversation with his first real TV ad; this one will appear in Iowa and remind caucus goers that the candidate wants to move them down the calendar. "Julián Castro has the courage to tell it is," says one supporter in the spot.

POLL WATCH

2020 vote for Congress (NBC News/WSJ, 733 registered voters)

Democratic: 49%
Republican: 42%

The premise of Republican advertising against impeachment, and a lot of the party's messaging, is that the Democrats who signed off on it were writing political suicide notes. But we haven't seen any evidence of the impeachment drama moving numbers from where they were this summer, with Republicans trailing the generic ballot outside the margin of error. The seven-point margin here is exactly what it was before hearings began and, more ominously for Republicans, exactly what it was in the weeks before the 2018 midterms. Most swing seats are a bit more Republican-leaning than the country at large.

ON THE TRAIL

LOS ANGELES – Jennifer Barbosa walked from door to door, dropping off fliers with a message most Angelenos did not want to hear: She could help rid them of Rep. Adam B. Schiff (D-Calif.). An independent who did not vote in the 2016 election, she was offering to replace the congressman who'd presided over the key hearings in the impeachment of President Trump with someone who might, for example, pay more attention to the problem of undelivered mail.

"Hundreds of constituents have contacted them about it, but they're not addressing it," Barbosa said, referring to Schiff's office. "Now, contrast that with — I know people sometimes roll their eyes when this is brought up — contrast this with how some Russian pranksters contacted his office and said, we have naked pictures of Trump. If I said I had some revenge porn about Trump, I could get in touch with Adam Schiff right away."

The reception, in one of the district's less liberal neighborhoods, was mixed — one woman thanking her for running, one saying she worked with Schiff and had no interest in voting against him. Since flipping a formerly Republican seat in 2000, in a race seen as a referendum on the then-congressman's vote for Bill Clinton's impeachment, Schiff had been drawn into a district that backed Hillary Clinton's 2016 bid by 50 points.

Schiff, a frequent target of the president, is not sweating his reelection. None of the people most strongly identified with impeachment are. House Speaker Nancy Pelosi's only significant challenge is from the left, as is House Judiciary Chairman Jerry Nadler's. Rep. Maxine Waters of California, one of the Democrats most often attacked for her impeachment advocacy, was set to face a perennial Republican candidate who'd raised more than $400,000 for his bid. He might be brought down by stalking charges, which would put Waters against another long-shot Republican with even more money.

While the threat of impeachment boosted the president's fundraising, opposition to it is inefficiently distributed. Republican admakers have targeted the 29 House Democrats who backed at least one article of impeachment while representing districts won by Trump, but the president's campaign sells anti-Schiff merchandise, not anything aimed at the swing districts. A president who can make even average citizens infamous, with a tweet, has not done much personally to target the Democrats seen as most vulnerable.

Some of those Democrats have faced local opposition, but not all; protesters, some organized by the Republican Party, were reportedly rowdy but outnumbered at events for Rep. Elissa Slotkin of Michigan and Rep. Abigail Spanberger of Virginia. And recent history hasn't found that impeachment, long before an election, moves many votes.

Democrats gained five seats in 1998, after Republicans — unlike the 2018 Democrats — spent much of their midterm campaign focused on the need to impeach the president. The party gained just one seat in the 2000 election, as Republican presidential nominee George W. Bush pivoted the party away from direct attacks on Clinton and toward a sense that presidential scandals were limiting Washington’s ambition.

“An era of tarnished ideals is giving way to a responsibility era,” Bush said in his 2000 speech at the Republican National Convention. “Our country is ready for high standards and new leaders.”

Bush’s coattails were short, with Democrats picking up five Senate seats, enough to split control of the chamber. On the House map, Democrats gained one seat in Arkansas, five in California, one in Oklahoma, one in Utah, one in Washington. Republicans gained one seat in Connecticut, one in Michigan, one in Minnesota, one in Missouri, one in Pennsylvania, one in Virginia, one in West Virginia; they won a seat in New York City’s suburbs as Democrats lost a seat there, and they effectively gained one more seat in Virginia when a conservative Democrat became an independent.

The new House majority is larger than the ones Republican were working with during the Clinton impeachment. But there’s been a collapse in “ticket splitting” for federal elections, and the GOP’s 2020 strategy is premised on it. While a few 2018 Republican challengers have put up impressive fundraising numbers, nearly all have been outraised by new Democratic incumbents. 

Frustratingly for the Republican Party, a number of the biggest fundraisers have been in high-profile seats that won't be competitive. Challengers to Rep. Alexandria Ocasio-Cortez (D-N.Y.) have raised close to $1 million combined; challengers to Rep. Antonio Delgado (D-N.Y.), whose Hudson Valley district is more competitive, have raised less than $60,000.

2020

Bernie Sanders. He won the endorsement of People's Action, a left-wing organizing coalition that vetted Democratic candidates and held a multi-round membership vote to determine who it supported. In the end, around 75 percent of the coalition's voting members backed Sanders, and some affiliates (such as New Hampshire's and Vermont's Rights & Democracy) had already been in his corner. 

Joe Biden. He unveiled scores of endorsements, mostly from faith leaders in South Carolina (which holds a pivotal primary) and Indiana (where Pete Buttigieg has struggled to gain "favorite son"-level support). He also picked up support from New Hampshire House Speaker Steve Shurtleff.

Elizabeth Warren. She's on the cover of Rolling Stone this week, with a friendly interview as part of the package.

Andrew Yang. He's rallying in Los Angeles with actor and rapper Donald Glover, a new supporter of his campaign.

Tom Steyer. He hired Zack Davis, formerly a senior adviser to Kamala Harris in Iowa, to guide his own campaign in the first caucus state.

Mike Bloomberg. He told NBC News that no other Democrat has the relevant experience he'd bring to the presidency; Joe Biden, he explained, had "never been the manager of an organization" and "never run a school system," despite the premise that he could begin fixing the country on day one. "The presidency shouldn't be a training job," Bloomberg said. "You need somebody who comes in and knows how to run an organization."

Tulsi Gabbard. She cast the lone "present" vote on the two articles of impeachment voted through the House, explaining in a statement that she "could not in good conscience vote either yes or no." In a follow-up video, she said she'd cast a vote "for much needed reconciliation and hope that together we can heal our country to usher in a bright future for the American people, our country and our nation."

Michael Bennet. His complaints about debate rules got a supportive op-ed from former New Hampshire Democratic Party chairwoman Kathy Sullivan; he's spending the two final campaign days before Christmas in central Iowa.

Deval Patrick. He released his 11-page "Deval for All" governing agenda, putting him roughly in line with the Democrats' House majority on policy: for immigration reform but against breaking up U.S. Immigration and Customs Enforcement (ICE), for raising corporate taxes but not to pre-2017 levels.

WHAT I'M WATCHING

For months, Democrats braced themselves for bad news. Conservative judge Reed O'Connor had sided with the Trump administration on a flimsy-looking lawsuit, which argued that the Affordable Care Act became unconstitutional when the 2017 tax bill reduced its individual mandate penalty to $0. A conservative-leaning panel on the U.S. Court of Appeals for the 5th Circuit reviewed that case for months, and if it sided with O'Connor, it would tear out the entire 2010 law on the premise that the ACA did not work without a mandate.

But yesterday, that three-judge panel punted. While the conservative majority agreed with the lawsuit's premise, it ruled that O'Connor, who has gained a reputation for ruling in conservatives' favor, struck down the law without doing "the necessary legwork of parsing through the over 900 pages of the post-2017 ACA, explaining how particular segments are inextricably linked to the individual mandate.”

That unusual decision put the lawsuit on two tracks, with the law's defenders working to get it before the Supreme Court quickly and O'Connor, whose initial opinion argued for scrapping the law, to explain what needed to go. Democrats weren't expecting this, and some were actually optimistic for a clear ruling against them. Why? They expected the current Supreme Court to rule against the conservatives, and on a timeline (June 2020) that would elevate the law's salience right before the next election.

In the meantime, hours before a debate, campaigns are approaching the legal threats to the ACA differently. Opponents of Medicare-for-all argue for tying the party closer to the ACA, which is popular, and making arguments for expanded insurance access by arguing that Republicans would undo their progress. Supporters of single-payer health care prefer an end-run around the whole legal fight: It was the mandate, first proposed by conservative wonks, that Republicans focused on in two lawsuits to destroy the law. Expanding Medicaid and Medicare to cover everyone could be done with decades of precedent, with no court ready to argue that a government program currently enjoyed by people over 65 can't be sold to people under that age.

COUNTDOWN

... 26 days until the seventh Democratic debate
... 46 days until the Iowa caucuses
... 54 days until the New Hampshire primary
... 65 days until the Nevada caucuses
... 73 days until the South Carolina primary