When a dozen Democratic presidential candidates gather for the fourth debate of the primary contest on Tuesday night, they’ll be looking at a campaign landscape that has, for the first time since the race started in earnest, changed substantially.

Sen. Elizabeth Warren of Massachusetts has pulled virtually even with Joe Biden, according to polling averages, challenging the former vice president’s long-held status as the Democratic front-runner. Biden has been dragged into the impeachment storm surrounding President Donald Trump, who has relentlessly leveled unsubstantiated allegations of corruption against Biden and his son. Sen. Bernie Sanders of Vermont, meanwhile, has slipped to a clear third place and, while still formidable, faces questions about his path forward after a heart attack.

Meanwhile, a new face with little support but lots of money, billionaire businessman Tom Steyer, will join the debate stage for the first time, adding another name to a trailing pack trying to gain ground on a top tier that has so far welcomed only Biden, Sanders, and Warren.

With impeachment dominating the national political scene and the Iowa caucuses less than four months away, here’s what we’ll be watching when the candidates take the stage at 8 p.m. (Philadelphia time) at Otterbein University in Ohio.

Impeachment in the debate spotlight

Tuesday’s debate will be the first since House Democrats formally initiated an impeachment inquiry over Trump’s attempts to press Ukraine to investigate Biden. Ever since, Democratic candidates have had to fight harder for voters’ attention, especially candidates outside the top tier.

While the impeachment inquiry has broad Democratic support, the candidates differ on whether they have seen enough to justify removing Trump from office. Even small differences have led to some sharp exchanges in previous debates, and questions about impeachment could create another wedge.

Some Democrats have come to Biden’s defense. But if anyone wants to challenge Biden’s argument that he’s the most “electable” person in the field, Trump’s verbal assault on Hunter Biden’s business dealings may have opened a soft spot — albeit one that could require echoing a president the party despises.

Will Warren be a target?

Warren is the only candidate who has enjoyed a steady and sustained rise in the polls, and she’s now been on an upward trajectory so long that some political insiders see her as the most likely Democratic nominee. A national Quinnipiac University poll released Monday found Warren gaining ground with both “very liberal” voters (who make up much of Sanders’ base) and white voters without college degrees (who make up much of Biden’s support).

» READ MORE: Elizabeth Warren builds momentum, energy to challenge Joe Biden, Bernie Sanders

Warren has faced relatively few direct attacks from other Democrats even as she has moved to the front of the pack. The reward for jumping to (or near) the top of the polls, though, is usually increased scrutiny from rivals.

Some, such as Mayor Pete Buttigieg of South Bend, Ind., have begun challenging the feasibility and costs of Warren’s big, liberal policy plans. More broadly, there’s an ongoing argument about whether her brand of politics can win in key general election swing states such as Pennsylvania, Michigan, and Wisconsin.

Any attacks come with risk: Warren is popular with many liberals, and a quick-witted debater who isn’t afraid to hit back.

How will Biden respond to Trump?

As Trump continues attacking Biden in TV ads and on Twitter, some have questioned whether the former vice president has responded forcefully enough.

Biden called for Trump’s impeachment for the first time last week — months after Warren did — and the Biden campaign is now airing ads in the early voting states calling Trump “unhinged.” But there’s some concern that Biden’s message isn’t breaking through.

The debate will provide a platform to go directly at the president, who has pushed a false narrative that Biden tried to get a Ukrainian prosecutor fired to protect his son.

Biden has framed Trump’s attacks as evidence the president is afraid of facing him. Biden has also made the case that he can connect with moderate voters, like those in working-class parts of Ohio, better than candidates like Warren and Sanders.

» READ MORE: Pennsylvania is critical in 2020. Here’s how Trump could win or lose it.

We’re also watching for whether the other candidates come after Biden. In previous debates he was the clear front-runner and thus the most common target. Now, he’s tied with Warren and the recipient of Trump’s onslaught.

So far, most of the candidates have been quick to excoriate Trump, but not to explicitly defend Biden. While the Republican National Committee has ruthlessly blasted impeachment efforts, the Democratic National Committee, which is neutral in the primary, has had to navigate how to punch back without seeming to favor Biden.

Sanders’ first debate after a heart attack

Sanders had, up until last week, largely avoided questions about his age (78), even as Biden, who is just two years younger, did not. But Sanders’ heart attack last week sparked new scrutiny about his health and ability to campaign moving forward. His national support among Democrats slipped to 11%, down from 16% last week, the new Quinnipiac poll found.

» READ MORE: Julian Castro went there on Joe Biden. But are voters actually worried about his age?

Sanders is known for his passionate — often riled up — style. During a heated exchange on health care in the July debate, Rep. Tim Ryan of Ohio shot back at Sanders, “You don’t have to yell” (Ryan has since dropped out). Will Sanders be the same ol’ Bernie on Tuesday, or a more subdued version? His campaign has said he will take on a less rigorous schedule, though Sanders himself said he would continue to run “a very vigorous campaign.”

Can Kamala Harris — or anyone else — break through?

With 12 people sharing three hours of debate time (less with commercials), the window for a breakout moment is closing fast.

The top three candidates have remained in the lead since the first debate five months ago. In past debates, the moments that made headlines — Sen. Kamala Harris of California going after Biden, New Jersey Sen. Cory Booker’s comments on race, Andrew Yang’s announcement he’d give away money — have not led to sustained movement in the polls.

» READ MORE: How Cory Booker scored a big moment at the Democratic debate, and what comes next

Steyer will be introducing himself to voters for the first time on a debate stage. Rep. Tulsi Gabbard of Hawaii, who made the July debate and then failed to qualify for September, is slated to be back on stage Tuesday.