In this debate day edition: What to look for on the Houston stage, the lessons of North Carolina's urban/rural special election, and polls that will give Democrats even more to worry about.

To new readers and subscribers, welcome: You can watch with us tonight before and after the debate. My goal is to spend the next 24 hours not uttering the phrases “trade jabs" or "trade blows,” and this is The Trailer.

HOUSTON — We now can say something confidently about the Democrats' 2020 primary debates: They have not radically altered the trajectory of this race. Not yet.

Unlike the past two Republican primaries, when dynamite debate appearances led to surges for individual candidates, the June debate in Miami and July debate in Detroit mostly reaffirmed Joe Biden's lead and the strength of his two most left-leaning competitors. In an average of national polls — these debates have national audiences, after all — when the Miami debate wrapped, Biden was supported by 32 percent of Democrats, Sen. Bernie Sanders (I-Vt.) by 16.9 percent and Sen. Elizabeth Warren (D-Mass.) by 12.8 percent. When the Detroit debates were over, it was Biden at 32.2 percent, Sanders at 16.2 percent and Warren at 14.3 percent.

What do these three have in common? They have not been hunting for debate “moments,” especially not the kind that involve showdowns with top contenders. Detroit's tag-team debate, in which Warren and Sanders defended Medicare-for-all against a group of more centrist candidates, produced defining lines from Warren (“I don’t understand why anybody goes to all the trouble of running for president of the United States just to talk about what we really can’t do and shouldn’t fight for”) and Sanders (“I wrote the damn bill”), but the targets were candidates polling under 1 percent. And Biden has largely protected his lead while fending off attacks, making no surprise answers or moves. 

Based on the usual pre-debate reporting, gossip and slanders, here are the questions that will get answered in a few hours.

Who goes after Joe Biden and why? In the past two debates, two struggling Democrats came ready to attack the former vice president on his age and his old, less-woke political stances. Those two Democrats, Rep. Eric Swalwell (Calif.) and Sen. Kirsten Gillibrand (N.Y.), have since quit the race. While Biden sometimes says he wants to focus on the president, not his rivals, he's been perfectly happy to punch back at others in his party, with eight years of good Obama-era feelings giving him the high ground.

But all nine of the candidates facing Biden onstage tonight see a path to victory if they convince voters that the 76-year-old Biden can't hack it in a general election. Sen. Kamala D. Harris (Calif.); Sen. Cory Booker (N.J.); Sen. Amy Klobuchar (Minn.); South Bend, Ind., Mayor Pete Buttigieg; former Texas congressman Beto O'Rourke; and former HUD secretary Julián Castro: All of them have been waiting and waiting for the “generational change” argument to land.

Based on the debates so far, the worst way to make that case against Biden is to do it explicitly. All of these candidates are well-liked by Democratic voters; Biden's favorables have slipped slightly, but by better than 3-1 margins, those voters think he's the safest general election choice. Attacking Biden over his record hasn't helped his rivals; tonight, they may spend more time trying to get viewers to consider whether he can be trusted to stay on-target as the nominee.

What does Elizabeth Warren do under pressure? Plenty of pre-debate speculation has focused, rightly, on the years-long arguments between Warren and Biden. It was frustrating to many Democrats that those two candidates didn't get to meet in Detroit or Miami. Warren's criticism of the Obama-Biden years is the one widely shared on the left — it squandered opportunities out of caution and timidity — and Warren pointedly leaves Biden out of her stories of what the Obama administration did right.

But other candidates have given more hints of what they might do than Warren has. In the run-up to tonight, Biden's campaign pointedly emphasized that he had released 20 years of tax returns; Warren-watchers saw that as a shot at her legal work for corporate clients, which is not included in the 10 years of returns she has released. Other campaigns think Warren is vulnerable and has gotten a pass on that legal work, seeing it both as a way to puncture her image as a working-class saint and to burn away her time onstage with explanations and spin.

There's also a chance that rivals, or moderators, press Warren on how she could remain a Republican into the mid-1990s. Warren's allies are baffled when other campaigns highlight this; she's spoken about it at length before, and if the attack comes from Sanders, it would emphasize that she's now a Democrat and he isn't. And in the past 72 hours, some Biden allies, led by former Pennsylvania governor Ed Rendell, have called Warren a “hypocrite” for raising money from high-dollar fundraisers for her 2018 Senate bid, then rolling it over into a 2020 presidential run that eschews high-dollar fundraisers. The goal there is lifting the “hypocrite” charge high enough for moderators to grab it; Warren's response would probably be to emphasize how much money she's raised for fellow Democrats, a total of $11 million in the 2018 cycle.

Which Kamala Harris shows up? Even close allies of the California senator now admit that she stumbled in Detroit. She was unready for an attack from Rep. Tulsi Gabbard of Hawaii (who missed this stage but has a good chance at the next one), and she got tangled up in the details of her modified health-care plan. The problem wasn't so much that voters resented her attack on Biden; it was that the Harris who showed up in Miami looked like a candidate who could dismantle the president in a debate and the Harris who arrived in Detroit did not.

Since then, Harris has slid in early-state polls while positioning herself closer to the center of the party. Sanders's presence in the race and the resilience of his support has made it less useful for candidates to endorse his views. Most of his current support is locked in; the voters who believe the party must do no harm and run an “electable” candidate may be gettable. Harris has increasingly emphasized that she would not be an “ideological” candidate and would focus on plans and executive orders that could be implemented quickly; onstage, she could contrast that approach with the Warren/Sanders approach, reintroducing herself to swing Democrats.

Do candidates pose new litmus tests or pivot to President Trump? After the Miami debate in June, Republicans crowed that Democrats were doing themselves permanent damage by taking so many left-wing stances. Democrats, being instinctively nervous, worried that the GOP was right. Since then, the president hasn't gained anything in head-to-head polls with Democrats, but the party's nerves are still jangling. Polling has found Democrats willing to support candidates they don't agree with if they think that other voters will agree with them. But apart from the also-ran candidates attacking Medicare-for-all, nobody has used the debates to explicitly argue against a liberal litmus test.

Some of the tests that have emerged since Miami are an assault weapons ban (which every candidate supports) and an assault weapon buyback (which not everyone supports). Biden has focused less on the newest idea than on emphasizing how he, alone onstage, got gun control laws passed when the NRA opposed them. He's also effectively escaped rounds about immigration reform by saying what's obvious to Democrats: The immigration policies they hate the most started with Trump. One day after the president's legal victory against asylum claims, there's a big opening for Democrats to shift questions back to him.

What do the moderators care about? Campaigns will hint at their onstage strategies, if it suits them. But the debate hosts never dare. All we knew ahead of the Detroit debate was that CNN would avoid asking “show of hands” questions. That didn't save Democrats from acrimonious, multi-round arguments about whether their plans would throw the election. And ABC News has not said anything about what questions could be asked or how. But as MapLight's Andrew Perez points out, the past few weeks of political commentary and questions on the network have been extremely skeptical of Medicare-for-all and of policies that old Democratic hands consider too radical in a general election.

READING LIST

“Warren says her tax plan asks just ‘two cents’ of the super-rich. But how much of a hit would Gates, Walton and their peers actually take?” by Annie Linskey

How the senator's signature policy, if implemented, would hit the superwealthy.

“Team Trump wrestles with its QAnon problem,” by Will Sommer and Asawin Suebsaeng

What do you do when some of your most fervent and meme-happy fans are conspiracy theorists?

“Republicans eke out a House win that gives them reason for worry,” by Michael Scherer and Mike DeBonis

The complicated lessons from a redo election.

“The man Trump once called ‘my African American’ leaves Republican Party,” by Yamiche Alcindor

Swing voters pop up in the most curious places.

“Crisis simulation? Professor moderators? With presidential debates, Democrats don’t buck tradition,” by Isaac Stanley-Becker

The new rules and tricks of debate prep.

TURNOUT WATCH

Republican legislator Dan Bishop won the race for North Carolina's 9th Congressional District, 10 long months after the midterm election for that seat. In 2018, Democrat Dan McCready won 48.9 percent of the vote, coming up fewer than 1,000 votes short in a race marred by Republican ballot fraud. Tens of millions of dollars and a new opponent later, McCready lost with 48.7 percent of the vote.

The president's party took a victory lap Wednesday, crediting the president's Monday night rally in Cumberland County for Bishop's win and for firing up the GOP base. Republicans also heavily outspent Democrats, with the Congressional Leadership Fund and the National Republican Congressional Committee syncing up their message: the Democrat was “greedy Dan McCready,” a man whose small-business story (he invested in solar energy) was a sham. And while McCready heavily outraised Bishop, the GOP's new WinRed portal was worth around $300,000 in donations to Bishop, most of it from out-of-state conservatives who got clued into the race.

Still, turnout for both parties fell from the 2018 election; Republican turnout just fell by less. The initial count gave Bishop 96,081 votes and McCready just 92,144 votes. Compared with last year, the GOP won 43,165 fewer voters; McCready won 46,197 fewer votes. In Cumberland County, last year's 1,138-vote win for McCready turned into a 36-vote win for Bishop. But overall turnout in the county fell by more than 40 percent, from 26,166 total votes to 14,991 total votes.

What really helped Bishop? First, support for the Libertarian Party completely collapsed. Last year, 5,130 voters cast ballots for Libertarian nominee Jeff Scott, putting McCready in striking distance of a win. But this year, Scott pulled just 767 votes, while a Green Party candidate won 371 votes. 

Second, Bishop focused on voters ignored by the last Republican nominee: the 55,000-odd members of the Lumbee tribe. Like McCready, Bishop favored federal recognition of the tribe and worked to introduce himself to their voters. In Robeson County, where most Lumbee tribe members vote, McCready's 2018 margin of 4,728 votes shrank to just 231 votes. Bishop won by less than 4,000 votes — Robeson, all by itself, saw enough of a shift to seal Bishop's victory.

But the end result of those shifts was an election that lined up closer to the 2018 midterms than the 2016 election. The 9th District, one of the state's tight Republican-drawn gerrymanders, was designed to rule any Democrat out of competition; Donald Trump carried it by 12 points. But since then, the suburbs of Charlotte, which produced the state's most recently elected Republican governor and senator, have been racing to the left. Last year, McCready won that Mecklenburg County part of the district by 9.5 points; this week, he took it by 12.4 points.

Still, the race between McCready and Bishop isn't easy to scale up to the presidential level. Bishop ran as a Trump loyalist; McCready, a veteran who emphasized his service in TV ads, pointedly opposed Medicare-for-all and criticized Minnesota Rep. Ilhan Omar by returning a donation from her. The party's 2020 nominee is likely to run to McCready's left, but his blueprint — organizing in left-behind rural counties and running up a score in the suburbs — is going to get plenty of second looks.

POLL WATCH

Do you have a favorable/unfavorable impression of this candidate? (NPR-PBS-Marist, 1,317 adults)

Elizabeth Warren — 40/40
Joe Biden — 44/45
Kamala Harris — 29/40
Bernie Sanders — 40/53

It's remarkable to think about it now, but in 2008, voters went to the polls with favorable impressions of both Barack Obama and John McCain. The election of 2016, with both major party candidates carrying unfavorable ratings above 50 percent, was unique. But this is the latest in a string of polls that find every Democratic candidate underwater; negative polarization has been working faster than positive polarization. In the Midwest (a polling region that includes both swing states and partisan strongholds), Warren is underwater by six points, Biden by eight points, Sanders by 18 points and Harris by 24 points. The president's approval rating across the region is negative 11 points.

Do you worry that the 2020 Democratic nominee will be too liberal, or not liberal enough? (CNN-SSRS, 908 registered voters)

Too liberal — 49 percent
Not liberal enough — 41 percent

This is a fascinating way to ask about the primary's defining question: Who's electable? By a one-point margin, Democrats themselves are more worried that the party will nominate a candidate who is not liberal enough for them; even 32 percent of self-identified liberals worry that the nominee will be too liberal. In other words, a substantial number of Democrats fret that the party might nominate someone who agrees with them, because they assume such a candidate would be weaker in a general election.

2020

Elizabeth Warren. On the morning before today's debate, she released an updated Social Security expansion plan, in which the wealthy would pay higher taxes to fund a $200 increase in payments; she also added a Medicare-for-all section to her website, after a good deal of left-wing skepticism about her commitment to the policy.

Kamala Harris. She scored the endorsement of Rep. Ruben Gallego of Arizona, who'd been a co-chair of his friend Rep. Eric Swalwell's campaign and who has emerged since 2016 as a leading, punchy voice for Democrats on national security and immigration.

Bernie Sanders. He'll head from the debate to Nevada, then South Carolina, for a series of events on student debt and health care.

Pete Buttigieg. He's returning to South Carolina on Monday and Tuesday, then making a first public campaign visit to Louisville

Tulsi Gabbard. She'll spend Friday through Sunday holding town halls in Iowa.

Tim Ryan. He'll be speaking at the University of New Hampshire tonight, while 10 of his rivals are on a debate stage.

Michael F. Bennet. He's holding town halls and meet-and-greets across Iowa from Friday through Sunday.

Donald Trump. His campaign will fly a banner over Houston during the debate tonight, reading “SOCIALISM WILL KILL HOUSTON’S ECONOMY! VOTE TRUMP 2020.” (The banner will not be visible inside the debate hall.)

COUNTDOWN

... four days until the Galivants Ferry Stump in South Carolina
... nine days until the Polk County Democrats' steak fry in Des Moines
... 50 days until Iowa Democrats hold their “Liberty and Justice Celebration” in Des Moines