The Democratic debate Thursday will feature all the top polling candidates on the same stage for the first time, with frontrunners former Vice President Joe Biden and Senators Bernie Sanders of Vermont and Elizabeth Warren of Massachusetts taking center stage and vying to be the center of attention.

Higher thresholds for polling and fundraising cut the list of candidates who made the stage Thursday in half, a departure from the previous two rounds of debates, which featured 20 candidates spread over two nights. This debate will offer a first glimpse at a narrowed-down primary field , with a clear top-tier and several other candidates still hoping for a breakthrough moment.

Will Warren and Biden challenge each other?

The most anticipated aspect of the matchup may be Warren’s approach to Biden. In the days leading up to the debate, Warren signaled she would not go after Biden directly. That approach has backfired for other candidates this cycle; Sen. Kamala Harris of California surged briefly after aggressively attacking Biden in the first debates, but her poll numbers have dropped since then. Warren may opt to stick to her strategy from the first debates, when she focused on her core message of reducing corporate power in the United States.

The main contrast between Warren and Biden may instead come down to how they go after President Donald Trump. In previous debates Biden has largely focused on Trump’s behavior in office, while Warren has argued that the president’s economic policies are helping the wealthy and hurting lower-income Americans. Their approaches will highlight how each Democrat would run against Trump in a general election.

How long will the Warren-Sanders partnership last?

Throughout the early stages of the 2020 race, the two liberal senators from New England have maintained that they’re close friends and avoided attacking each other on the stump. They didn’t clash when they appeared on the same debate stage for the first time in July, disappointing any viewers hoping for a showdown between the Democratic Party’s two biggest progressive stars.

That dynamic will likely continue Thursday. With nearly five months to go before the first primary votes are cast, it’s still early for Warren and Sanders to attack each other. The candidates also share similar views on health care, climate change and other key issues driving the Democratic primary race, leaving little room for policy disagreements. (In contrast, Warren and Biden have a long history of clashing on some economic policies). Gender is one of the only real dividing lines between the two, but so far Warren has not made running as a woman central to her campaign.

But Sanders and Warren’s supposed partnership can’t last forever. At some point they’ll have to differentiate themselves. When that happens, it will be interesting to see how the process unfolds.

Progressives will outnumber moderates (by a wide margin)

Even if Warren and Sanders lay off Biden, the optics of the party’s progressive stars flanking the moderate frontrunner will be hard to ignore. Biden will be surrounded — and outnumbered — on stage by Democrats who are far more liberal than he is. In addition to Warren and Sanders, the former vice president will have to contend with other liberal rivals, including Harris and Sen. Cory Booker of New Jersey, who have challenged him in the previous debates.

Of the 10 candidates on stage, eight have positioned themselves as progressive candidates: Warren, Sanders, Harris, Booker, South Bend Mayor Pete Buttigieg, former Obama cabinet official Julián Castro, former Texas congressman Beto O’Rourke and entrepreneur Andrew Yang. Biden and Sen. Amy Klobuchar of Minnesota will be the only candidates at the September debate who are running on more moderate, center-left platforms.

In previous debates, Biden tried to position himself as both progressive enough to appeal to the party’s liberal wing and moderate enough to beat Trump in a general election. The results were mixed, with Biden at times seeming unsure of how far to go in either direction. The task will be even harder Thursday, when Biden will have fewer moderates on stage alongside him.

Time is running out for the second tier

For several months, a group of second-tier candidates has been stuck in limbo between the three frontrunners and a third-tier of hopefuls struggling to crack one percent in the polls. Harris has consistently led the middle group in polling, and seemed poised after the first debate to join Biden, Warren and Sanders in the top tier, but has not yet made the leap. Buttigieg surprised the political establishment by leading the entire field in second quarter fundraising, but he hasn’t translated his cash advantage into a jump in the polls. The other members of the second-tier —

Booker, O’Rourke and Yang — have not budged much in the polls, either.

In such a crowded primary field, it’s difficult to stand out at all, let alone rise to the top of the pack. Name recognition has played a role in shaping early polling in the race, to the advantage of Biden and Sanders, who were both well known. Some candidates have also argued that their decision not to court wealthy donors or accept campaign contributions from corporations has put them at a disadvantage.

Regardless of the reasons, the second-tier candidates are running out of time to make an impression before voting begins in February. To remain credible, candidates like Harris and Booker need their fortunes to change fast because the primaries have already taken on the feel of a three-way race between the candidates at the top.

The race for VP

As the field narrows, the jockeying for position as a potential vice presidential nominee will begin in earnest. That’s especially true of candidates who have performed well in debates but haven’t gained any traction with voters in polls.

Castro falls into that category. The former mayor of San Antonio and former secretary of the Department of Housing and Urban Development has emerged as an expert on immigration policy, and a leading voice in the primary field on issues of LGBTQ rights. Castro, 44, was reportedly on the short-list of Hillary Clinton’s VP picks in 2016, and would likely be a contender again next year, especially if an older candidate is the presidential nominee.

Biden has said repeatedly that if he won the nomination it “would be great” to have a female running mate. For her part, Harris has signaled she would accept a second-string spot on the Democratic ticket.

Of course, the formal vice presidential nominee selection process is still nearly a year away. But as the primary race heads into the fall, low-polling candidates will increasingly be viewed as auditioning for future roles in a Democratic administration. That process is already underway, even if it won’t be front and center Thursday in Houston.