The Democratic presidential primary has taken a back seat to the impeachment inquiry over the past few months, so it's fitting that the fifth candidate debate will take place on the same day that the most anticipated impeachment witness, Ambassador to the EU Gordon Sondland, testifies before the House Intelligence Committee.

But a lot more than the path to impeachment has changed since the Democratic candidates last gathered on the debate stage.

New candidates have entered — and semi-entered — the race, though neither former Massachusetts Gov. Deval Patrick nor former New York City Mayor Michael Bloomberg will be in Wednesday's debate. South Bend, Ind., Mayor Pete Buttigieg has taken Massachusetts Sen. Elizabeth Warren's place as the surging candidate of the moment. And after months of catering to the progressive activist wing of the party, more and more candidates are now focusing their pitches on moderate voters.

Here are four key questions ahead of Wednesday's debate:

1) Can Pete take the heat?

In October, Buttigieg was one of several candidates who repeatedly used their time to criticize Warren. "Your signature, senator, is to have a plan for everything, except for this," he memorably said about her lack of a detailed explanation to fund a "Medicare for All" health care system.

At the time, Warren was the candidate rising in the polls. Now, Buttigieg is moving up fast. The Des Moines Register's latest Iowa Poll shows Buttigieg with a commanding 9-point lead over Warren, former Vice President Joe Biden and Vermont Sen. Bernie Sanders. Other surveys also show Buttigieg polling well in New Hampshire, though struggling in the other two early contests with more diverse electorates: South Carolina and Nevada.

Biden and Warren have had their turns as the front-runner in every other candidate's crosshairs. Wednesday will likely be Buttigieg's night. How will the millennial mayor answer tough questions about his lack of statewide experience, his struggle to win support from African American voters and his opposition to a Medicare for All approach to health care that has become the signature issue for Sanders and Warren?

2) Can Warren get back on offense?

Warren has spent the bulk of the past month struggling to answer the critiques that Buttigieg and other rivals raised on the October debate stage: Would her health care plan raise taxes on the middle class? Why hadn't the "plans candidate" released the granular details on the issue that Democratic voters care about more than anything else? And just how committed was she to Medicare for All?

Warren finally released her plan on Nov. 1. It would notraise middle-class taxes — though many experts were skeptical of the mix of optimistic economic and political projections the proposal relies on to make costs work. (Among them: passing comprehensive immigration reform, drastically lowering health care costs and more efficient federal tax collection.)

But as Warren firmly and publicly committed to a comprehensive national health care system, more and more polls showed voters were hesitant to fully abandon their private health insurance.

And last week, Warren again sowed doubt about her ultimate health care goals by announcing that she'd tackle health care in two parts: an initial public option bill right away and then a more expansive single-payer measure in her thirdyear as president. Given that presidents rarely have the luxury of more than a handful of major legislative achievements (think tax cuts for President Trump and George W. Bush or the Affordable Care Act for Barack Obama), the announcement raised questions about whether Warren is fully committed to the plan.

More than any other candidate in the race this year, Warren has been able to set her own terms for the campaign. But a month of confusing and, at times, contradictory health care messaging has opened the door to a lot of criticism Wednesday night.

3) Will Biden be a factor? (Does he need to be?)

Before every debate, Biden's campaign has tried to set expectations with the same basic pitch: Other candidates with lower name ID and polling numbers would try to go viral by attacking the candidate at the top of the national polls, but Biden would ignore them and speak directly to the American people.

With so much of the focus on Warren last month, Biden went long periods without even speaking and didn't figure into many of the main narratives coming out of the night. And that seemed to be just fine with Biden and his campaign. While he has faded a bit in Iowa and struggled to raise money, Biden still enjoys commanding leads in South Carolina and Nevada, as well as sitting toward the top of national polls.

Biden has maintained his spot atop the field with a very non-2019 approach to politics: holding just a handful of relatively small rallies a week, doing a lot of big-donor fundraisers and generally avoiding any and all buzzy viral moments on social media. The slow-and-steady formula has worked well for Biden so far, and he may be content to once again stand to the side while other top-tier candidates take all the incoming criticism.

Or, he may go on offense against Buttigieg, who has peeled off some of Biden's more moderate supporters. This more than anything else will likely set the tone of Wednesday's debate.

4) Is the top tier now fully set?

The wide gulf between Biden, Warren, Sanders, Buttigieg and every other candidate remains. With fewer than 80 days remaining before the Iowa caucuses, the likelihood of one or two other candidates joining the top tier grows smaller and smaller.

But the most recent NPR/PBS NewsHour/Marist Poll shows that three-quarters of Democratic voters still haven't made up their mind. And as candidates like New Jersey Sen. Cory Booker and California Sen. Kamala Harris now remind voters at every campaign stop, both Iowa and New Hampshire are states that traditionally break very late in the race.

Harris, Booker and Minnesota Sen. Amy Klobuchar, among the other "lower tier" candidates, all face the same basic challenge: capture voters' attention Wednesday night with a viral moment or two that goes beyond a one- or two-day fundraising boon and news story. They need to change the way that voters think about them and create some long-term momentum that could lead to a steady rise in the polls.

The goal is simple, but it's hard to achieve and hasn't really happened with any previous debate. Harris made waves in the first debate by criticizing Biden's stance on federally mandated busing, but after a few weeks of momentum she dropped in the polls and never really recovered. Former Housing and Urban Development Secretary Julián Castro saw a less dramatic bump after a strong showing in the first debate, but he won't be on the stage Wednesday.

Copyright 2021 NPR. To see more, visit https://www.npr.org.