Leading Democratic White House contenders Joe Biden and Elizabeth Warren will share the stage for the first time on Thursday when the top 10 candidates for the party's presidential nomination meet in a debate in Houston.

Twenty Democrats continue to vie for their party's nomination to challenge Republican President Donald Trump in November 2020, but party rules about polling and fundraising resulted in only half of them qualifying for the debate in Houston.

All of the top 10 Democrats will debate on one night, allowing voters to take stock of the leaders side-by-side.

Here are some key moments to watch for during the debate. REUTERS/Lucas Jackson

THE TOP THREE:

The condensed debate stage will generate some first-time matchups, but arguably the most anticipated is the one between former Vice President Biden, the leader in most opinion polls, and U.S. Senator Warren of Massachusetts, who is nipping at his heels.

The two exemplify the ideological divide between the moderate and liberal wings of the Democratic Party, and the central question voters face in the 2020 primary race: Between pledges to restore pre-Trump normalcy, or embark on ambitious reform, what is a better ticket to defeat Trump in the November 2020 election? Pictured: Joe Biden speaks at the UnidosUS Annual Conference, in San Diego, California, August 5, 2019. REUTERS/Mike Blake

THE TOP THREE: Warren's recent gain on U.S. Senator Bernie Sanders of Vermont in polls has also raised speculation that the two friends and progressive allies could abandon their agreement to not criticize one another.

"In Detroit, those two somewhat teamed up to defend the progressive flag of the party. Given the state of the race today, I will keep an eye on whether Sanders decides to draw a sharper contrast with Warren," said Joel Payne, a Democratic strategist who worked for Hillary Clinton in 2016. Pictured: Elizabeth Warren speaks at a Town Hall in Los Angeles, California, August 21, 2019. REUTERS/Lucy Nicholson

THE TOP THREE: For U.S. Senator Kamala Harris of California and South Bend, Indiana, Mayor Pete Buttigieg, the debate presents a narrowing set of chances to break through into the top tier. Pictured: Kamala Harris speaks during the Presidential Gun Sense Forum in Des Moines, Iowa, August 10, 2019. REUTERS/Scott Morgan

THE TOP THREE: Buttigieg "needs a moment, he needs to engage in the way that people like and that shows that he's the kind of person who, for example, could take on Trump," said Bob Shrum, a political science professor at the University of Southern California and former Democratic strategist. Pictured: Pete Buttigieg attends the Iowa State Fair in Des Moines, Iowa, August 13, 2019. REUTERS/Eric Thayer

HOURS WITHOUT A GAFFE?

Biden enters the his third debate atop the field, even after his first two debate performances were criticized as lackluster, while his campaign has had to fend off questions about repeated misstatements on the campaign trail.

In the second debate in July, Biden fully embraced his role at the race's centrist, criticizing his opponents for pulling the field to the left.

At that time, Biden benefited from the chorus of other moderates joining his defence. But now, those candidates are off the stage and Biden may have to make the centrist case himself.

Biden, 76, has also faced questions from some voters about his age and whether he has the stamina to make it through a lengthy and taxing presidential race. In Houston, he will be standing next to Sanders, 78, the oldest candidate in the field.

"Biden will likely continue to take a page from Donald Trump in 2016. If you'll remember, Trump essentially, just treaded water in the debates," said Payne.

"He never won any debate but he managed to stay above the fray. Biden is likely hoping that he can ride a similar strategy to get through a splintered Dem primary." Pictured: Joe Biden speaks on the second night of the second 2020 Democratic U.S. presidential debate in Detroit, Michigan, July 31, 2019. REUTERS/Lucas Jackson

WHAT DOES SHE STAND FOR?

The candidate who has benefited the most from the debates so far is Harris - but after a well-reviewed performance in the first round, she watched the lead she amassed dwindle by the second debate when she pulled off a less-than-stellar showing. Heading into the third debate, she is struggling to break through to the top of the field.

A September Reuters/Ipsos poll found that Harris and Buttigieg were tied with 4% support of Democrats and independents - a big drop for the senator, who once registered at 10%.

In the first debate she attacked. In the second debate she was attacked. Now, she is under pressure to articulate what she stands for.

"In the second debate she was not very effective when she was challenged. I myself can't quite figure out what her message is other than I was a prosecutor and I can prosecute Trump -- which is a pretty thin message," Shrum said. Pictured: Kamala Harris gestures on the second night of the second 2020 Democratic U.S. presidential debate in Detroit, Michigan, July 31, 2019. REUTERS/Lucas Jackson

CLOSING WINDOW

For the candidates in the bottom half, such as former U.S. Housing Secretary Julian Castro, U.S. Senators Amy Klobuchar of Minnesota and Cory Booker of New Jersey and business leader Andrew Yang, the debate provides one of their only chances to try to change the public perception of their candidacy.

One way to do that is to try and land a blow on leading rivals, such as when Harris effectively attacked Biden over his record on race at the first debate in Miami.

But ultimately, since most of these low-polling candidates have positioned themselves between the Biden-represented centrist establishment and the left wing led by Warren and Sanders, they will struggle to gain momentum unless Biden declines, experts say.

The window is closing, Shrum warns.

"For those folks to break through, Biden has to fade and if he's going to fade, he has to start doing this in this debate because he has shown incredible resilience," Shrum said. Pictured: Julian Castro talks with Cory Booker as Amy Klobuchar walks past during a break at the first U.S. 2020 presidential election Democratic candidates debate in Miami, Florida, June 26, 2019. REUTERS/Mike Segar

DUELING OVER HEALTHCARE

What the nation's healthcare system should look like has proven to be the most divisive topic during the first two debates.

That could remain the case on Thursday, after the latest government report showed on Tuesday that the share of Americans without health insurance rose for the first time in a decade last year.

About 27.5 million Americans, or 8.5% of people, did not have health insurance in 2018, an increase of almost 2 million from the year before when 7.9% of people lacked coverage, the Census Bureau report found.

The most liberal candidates like Warren and Sanders are advocating a complete government takeover of health insurance, a system that would eliminate private insurance and raise taxes.

But the more centrist wing of the party, led by Biden and joined by Klobuchar, say that a public option should be available to whoever would like to have it but should not be mandated. Biden argues that the national electorate is not interested in a government takeover of health insurance. Pictured: Participants hold signs as Bernie Sanders speaks at a news conference to introduce the "Medicare for All Act of 2019" on Capitol Hill in Washington, April 10, 2019. REUTERS/Aaron P. Bernstein