Six candidates will take the stage on Tuesday night for the first Democratic presidential primary debate of 2020 — and the last one before the Iowa caucuses.

The debate, which will take place at Drake University in Des Moines, will be one of the last chances for the primary field’s top contenders to stand out.

Four candidates — Sen. Bernie SandersBernie SandersBiden most trusted among Democratic primary voters on foreign relations: poll Warren: Sanders said a woman could not win the White House Hill.TV’s Saagar Enjeti warns Biden’s Iraq record could be general election issue MORE (I-Vt.), Sen. Elizabeth WarrenElizabeth Ann WarrenBiden most trusted among Democratic primary voters on foreign relations: poll Warren: Sanders said a woman could not win the White House Conservatives slam Warren’s call to put transgender women in women’s prisons MORE (D-Mass.), former South Bend, Ind., Mayor Pete ButtigiegPeter (Pete) Paul ButtigiegBiden most trusted among Democratic primary voters on foreign relations: poll Warren: Sanders said a woman could not win the White House Sanders-Warren fight unnerves progressives MORE (D) and former Vice President Joe BidenJoe BidenBiden most trusted among Democratic primary voters on foreign relations: poll Russia hacked Ukrainian gas company at center of impeachment inquiry: report Warren: Sanders said a woman could not win the White House MORE — appear most likely to take the top spots in the Feb. 3 caucuses. But Sen. Amy KlobucharAmy Jean KlobucharPoll: Biden leads Democratic field nationally after Sanders takes top spot in Iowa poll Biden found leading in Iowa by Monmouth poll Sanders: Releasing list of Supreme Court picks ‘not a bad idea’ MORE (D-Minn.) is also banking on a strong finish in Iowa to buoy her presidential prospects.

Those five candidates, along with billionaire philanthropist Tom SteyerTom Fahr SteyerSanders: Releasing list of Supreme Court picks ‘not a bad idea’ It’s time for the Democratic candidates to talk more about national security Buttigieg picks up Iowa congressman’s endorsement ahead of caucuses MORE, will make up the debate roster on Tuesday.

Here are five things to watch as the candidates take the stage in Des Moines:

1. Will Sanders take heat?

Sanders has proven himself to be one of the most durable candidates in the Democratic primary field. But after a Des Moines Register–CNN poll released on Friday showed him taking the lead in Iowa, with 20 percent support among likely Democratic caucusgoers, Tuesday’s debate will be the first in which Sanders is the front-runner.

So far, his top rivals have largely avoided direct confrontations with the progressive firebrand, aware of his deep support among the Democratic Party’s activist factions. Now that he’s polling at the top of the pack, however, he could see an onslaught of criticism when he takes the stage in Des Moines.

He may also face questions about a CNN report detailing allegations that Sanders told Warren during a private meeting in 2018 that he did not believe a woman could win the White House. Sanders denied that allegation in a lengthy statement to the news network. But it could provide fodder for his rivals.

2. Will the Sanders-Warren feud come to a head?

Sanders and Warren played nice over the course of 2019, seeing one another as a key ally in the Senate and on the campaign trail, where they both occupy the progressive lane.

But that nonaggression pact fractured this week after Politico reported that Sanders’s campaign had begun quietly directing volunteers to attack Warren as a candidate of the elite. In response to the news, Warren told reporters on Sunday that she was “disappointed to hear that Bernie is sending his volunteers out to trash me.” Sanders denied any responsibility in the matter.

Tensions escalated further on Monday after CNN reported that, during a private meeting in December 2018, Sanders told Warren that he did not believe a woman could win the presidency. He denied that allegation, but Warren confirmed the report in a statement, recalling how she told Sanders that she “thought a woman could win.”

“He disagreed,” Warren said.

All that sets the stage for a potentially explosive confrontation on Tuesday night.

3. Can the moderates make their case?

With the release of the Des Moines Register poll on Friday, the Democratic primary field’s moderate contenders appear to be in an uneasy position. Buttigieg, who led the pack in Iowa in a Register poll from November, fell 9 points into third place, while Biden and Klobuchar remained stagnant at 15 percent and 6 percent support, respectively.

Meanwhile, Sanders ticked up 5 points into first place, while Warren gained 1 point, maintaining her No. 2 standing in Iowa.

The three moderates on the debate stage will be under pressure to stand out on Tuesday night and leave a mark on voters before the caucuses on Feb. 3. They could also use the forum to more aggressively attack the leading progressives as too divisive or unrealistic in their sweeping policy proposals.

4. Does Biden’s Iraq War vote come under scrutiny?

The soaring tensions between Washington and Tehran that emerged following the death of Iranian Gen. Qassem Soleimani earlier this month in an American drone strike has thrust the issues of foreign policy and U.S. military involvement in the Middle East back into the spotlight.

Biden is the only presidential candidate who voted in favor of a 2002 measure authorizing the use of military force in Iraq — a vote that is now seen in as a liability among key elements of the Democratic Party.

Sanders, in particular, has hammered Biden over the 2002 vote, accusing him of helping lead the U.S. into a protracted, aimless and expensive military campaign. And it remains possible, if not likely, that Biden will face a barrage of criticism when he takes the stage on Tuesday.

Biden has since called that vote a “mistake.” But his rivals for the Democratic nomination — Sanders, in particular — see his record as an easy target that could weaken the longtime front-runner.

5. Can Buttigieg mount a comeback?

A couple of months ago, Buttigieg appeared to be the candidate to beat in Iowa. Now, with the caucuses just three weeks away, his chances appear somewhat less certain, and the debate will be a chance for him to climb back to the top.

But he still faces some risks. His fundraising practices have been a target of criticism at past debates, including in last month’s forum in Los Angeles, when Warren excoriated him for holding a high-dollar fundraising event in a “wine cave” in California.

And issues surrounding race relations with the African American community in his hometown of South Bend, where he was mayor for eight years, has proven to be a sore spot for Buttigieg on the campaign trail.

Still, Buttigieg has shown himself to be resilient in the race — he worked his way into the top-tier of candidates after entering the primary contest last year as a virtual unknown outside Democratic political circles — and whether he can kick off a new surge on Tuesday is a central question heading into the debate.