In this special post-debate edition: What changed for Democrats last night, what ads Iowans are seeing now and where the candidates are going next.

That debate was the last truly big event for the candidates until the Iowa caucuses. It's a good time to answer questions about what's happened and what happens next: Please send those questions to david.weigel@washpost.com.

If I spent $120 million to get onto a debate stage, I'd probably want to eavesdrop more than Tom Steyer did, and this is The Trailer.

DES MOINES — Before looking at what happened at Tuesday night's debate, consider what the Democrats were doing and saying one week earlier. 

Sen. Elizabeth Warren (D-Mass.) released a bankruptcy plan seen as a challenge to former vice president Joe Biden, who pushed through a 2005 law that introduced Warren to electoral politics. Sen. Bernie Sanders (I-Vt.) introduced legislation to prevent any unilateral presidential action against Iran. And that debate, about what came next in Iran, had started to clarify the different roles that Biden, Warren and Sanders saw for American military power.

These were substantive arguments, the kinds Democrats had been having for most of this campaign. So how did the party's seventh primary debate turn into a fight about whether one candidate turned down a handshake? How did reporting on a 2018 conversation between Sanders and Warren change the race?

The Warren-Sanders alliance is dead. Until this past weekend, Sanders and Warren had a nonaggression pact with little precedent in the history of contested primaries. As Sanders has said, he urged Warren to run for president in 2015, when there was a draft movement to make that happen. Both candidates were (and are, though that could be changing) widely loved on the political left, so much so that some supporters urged them to stay in the race, even if one of them fell behind, to accrue delegates.

But in October, the feelings among the candidates’ supporters — not the candidates themselves — began to change. Sanders slowly recovered from a summer slump, and the new institutions of the resurgent left portrayed Warren as an updated Hillary Clinton: an establishment Democrat who could not be trusted. The Medicare-for-all fight exemplified this. Both senators’ allies in Congress were happy with Warren’s alternative to the Sanders transition plan. Sanders’s supporters called it proof that Warren would never deliver on a working-class agenda and would buckle whenever capital told her to.

Sanders and Warren are different, with overlapping politics but divergent theories of how to win. Warren sought to consolidate as much left-wing activist support as possible while winning over elected Democrats. Sanders had a different theory: He could change the makeup of the primary electorate by mobilizing activists and non-voters who could overwhelm the old party establishment. Warren, a “capitalist to her bones,” offered the party a populist software update; Sanders offered it the chance to join his movement, or lose. 

By December, Sanders’s strategy looked more effective. It also had an internal tension that Warren, this week, decided to exploit. Sanders, who has rejected offers to run as an independent or third-party candidate, had supporters who despised the Democratic Party. He spent years campaigning for Democrats and working to change their primary rules, reducing the voting power of “superdelegates.” But his loyal, organized base included plenty of voices who did not like or trust the party and gleefully attacked Sanders’s rivals. Sanders himself pledged not to attack anyone else personally, while his supporters ripped into Warren for everything from an old appearance at a Federalist Society forum to some goofy dancing at campaign events.

With few or no votes to gain with the Sanders base, Warren began making more appeals to female voters — a clear majority in every Democratic contest — that had less to do with ideology. When Tuesday's debate was over, Sanders supporters were tweeting images of the refunds they were demanding from Warren, images of snakes, and hashtags like #WarrenIsASnake and #NeverWarren.

“This is horrific,” tweeted Charles Lenchner, the co-founder of People for Bernie, a grass-roots group that backed Sanders in both primaries. (The other co-founder, Winnie Wong, now works for Sanders.) “Please do NOT support this. It’s really not helping, and is the kind of thing that Russian bots and Trump social media might be amplifying.”

For many Democrats, anything that reminds them of the most intransigent Sanders supporters, the ones who said “Bernie or Bust” in 2016, makes it harder to reward him. And after Tuesday night, there's only one rival candidate sticking up for his approach: Rep. Tulsi Gabbard (D-Hawaii), who has largely moved on from Iowa and whose strength in polling comes largely from Republicans.

There is no “unity candidate.” The departure of Sen. Cory Booker (N.J.) from the race was expected and led to another round of Democratic griping about a debate stage with no candidates of color. It also meant that every Democratic senator who had tried to carve a path between Warren and Sanders was out of the race. Booker was the last of a trio of senators (along with California’s Kamala D. Harris and New York’s Kirsten Gillibrand) who had endorsed Medicare-for-all as just one of many health insurance plans. That looked tenable in 2018 and simply wasn’t once voters tuned in.

The failure of those candidates left six Democrats on the debate stage: Four who aren’t trusted by the left, and two who are viewed with terror by the party’s moderates. Tuesday's Medicare-for-all conversation demonstrated that there's no longer a compromise position between these candidates. Warren is aiming for it, as her defense of the USMCA trade deal showed, as she drew a contrast between her support for incremental deals and Sanders's insistence on fighting until the deal met his standards. But the past 72 hours have torpedoed either candidate's chances of unifying the party and allowed Biden, who has the strongest support of any centrist candidate, to look like the least divisive choice who could wrap up a primary quickly.

The hawks have left the building. All this drama around Sanders and his critics can obscure how much the party continues to move in the Vermont senator’s direction. This was the first debate since tensions with Iran began to escalate, the sort of situation that, years ago, might have found the candidates jockeying to sound the most ready to fight the War on Terror.

Things change. The debate’s long opening exchange about Iran put the most pressure on Biden, who continued to walk away from a vote that had looked politically astute 18 years ago: supporting regime change in Iraq.

“They said they were just going to get inspectors in,” Biden said. “The world, in fact, voted to send inspectors in and they still went to war. From that point on, I was in the position of making the case that it was a big, big mistake.”

Every Democrat argued for a lighter footprint in Central Asia and the Middle East, with former South Bend, Ind., mayor Pete Buttigieg speaking personally about what it felt like to watch soldiers kiss their kids goodbye and go to war. Warren, who had been looking for a chance to talk about this, emphasized that she'd seen “one general after another” mislead Congress about progress in Afghanistan: “Someone new comes in, and we've just turned the corner.”

Donald Trump isn't shaping this primary. Last June, the president hinted that he'd live-tweet the first Democratic debate, leading to a frothy news cycle about whether, as in 2016, he could suck the oxygen out of his rivals’ campaigns. 

Trump didn’t commit to the bit. He tweeted that the Democrats’ debate was “boring.” That was it. Since then, Trump has failed to make it into these debate news cycles, and he had special trouble doing so Tuesday. While the president rallied in Milwaukee, most news and online chatter focused on the debate. When Trump squeezed into the debate coverage, it was by reacting to what was already underway — he said that he believed Sanders over Warren in their dispute over whether Sanders suggested a woman would face disadvantages in a general election.

“I don't want to waste a whole lot of time on this,” Sanders said in Des Moines, when asked about the spat with Warren, “because this is what Donald Trump and maybe some of the media want.”

The president has scheduled a Jan. 30 rally in Des Moines, giving him another chance to elbow into primary coverage. But the disinterest in an impeachment question Tuesday night epitomized Trump's role in Democratic politics: They see a chance to get rid of him in 371 days, and they don't want to think about him unless they're thinking about that.

READING LIST

“Democrats clash over positions on war and peace — and gender in politics,” by Matt Viser, Michael Scherer and Annie Linskey

What happened last night.

“What the hell is Tom Steyer doing on that debate stage,” by Ben Smith

How the billionaire bought his way into the primary.

“The explosive question for Democrats: Can a woman defeat Trump in November?” by Sean Sullivan and Annie Linskey

The debate only some Democrats wanted.

IMPEACHMENT WATCH

The latest on the impeachment of President Trump

AD WATCH

Amy Klobuchar, “People.” The Minnesotan's ads have tended to focus on her, telling her story of electoral and legislative success. This one gives the microphone to Iowa voters, who are impressed that she visited all 99 counties and think it's “awesome” that she's passed so many bills.

Deval Patrick, “Deval for All.” The former Massachusetts governor is still introducing himself in early states, with the biographical pitch that made him a phenomenon 14 years ago: raised on the south side of Chicago, then fighting his way into tremendous success.

CANDIDATE TRACKER

Joe Biden. He was endorsed by Rep. Tom Malinowski of New Jersey, a freshman who had been an assistant secretary of state in the Obama administration but who (along with the state's entire federal delegation) had previously supported Sen. Cory Booker.

Bernie Sanders. He won the support of the Clark County Education Association, the first endorsement from a teacher's union for any candidate in Nevada, as national unions remain on the sidelines, and the endorsement of Make the Road Action, the latest immigrant rights group moved by his promise to put a moratorium on deportations.

Elizabeth Warren. She's returning to Iowa on Friday for the last full weekend of campaigning before members of the Senate are tied up with the impeachment trial.

Amy Klobuchar. Already running ahead of the field in Iowa endorsements, she got the support of Council Bluffs-area state Rep. Charlie McConkey, who'd previously backed Booker. 

Tulsi Gabbard. She's continuing to campaign through New Hampshire, with town halls each day: Derry on Wednesday, Manchester on Thursday.

John Delaney. He's continuing to campaign at intimate events in early states, in New Hampshire on Thursday, then back to Iowa after.

COUNTDOWN

... five days until the Iowa Brown & Black Forum 
... 13 days until the special legislative election in Texas
... 19 days until the Iowa caucuses
... 27 days until the New Hampshire primary
... 38 days until the Nevada caucuses
... 46 days until the South Carolina primary
... 49 days until Super Tuesday