(Reuters) - 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.

FILE PHOTO: Democratic 2020 U.S. presidential candidates stand on stage (L-R) author Marianne Williamson, U.S., Rep. Tim Ryan, U.S. Senator Amy Klobuchar, South Bend Mayor Pete Buttigieg, U.S. Senator Bernie Sanders, U.S. Senator Elizabeth Warren, former U.S. Rep. Beto O'Rourke, former Colorado Governor John Hickenlooper, former U.S. Rep. John Delaney, Montana Governor Steve Bullock on the first night of the second 2020 Democratic U.S. presidential debate in Detroit, Michigan, July 30, 2019. REUTERS/Lucas Jackson/File Photo

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

Here are some key moments to watch for during the third Democratic presidential debate, which follows the June debate in Miami and the July gathering in Detroit.

THE TOP THREE

Arguably, the most anticipated of the first-time matchups is 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 divide between the moderate and liberal wings of the Democratic Party, and the central question 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?

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.

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.

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.

HOURS WITHOUT A GAFFE?

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

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

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

After his rivals at the last debate in Detroit challenged his record as vice president to former President Barack Obama, Biden is prepared to defend Obama’s legacy, a Biden adviser said.

Unlike the last two debates where only a fraction of time was spent on debating foreign policy, more substantial discussions are expected this time given Trump’s “missteps,” the adviser added.

On Tuesday, Trump fired national security adviser John Bolton amid disagreements over how to handle foreign policy challenges such as North Korea, Iran, Afghanistan and Russia.

Biden, 76, has also faced questions about his age and whether he has the stamina for a lengthy and taxing presidential race. In Houston, he will stand 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.”

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, the gains she amassed dwindled by the second debate when she pulled off a less-than-stellar showing.

A September Reuters/Ipsos poll found Harris and Buttigieg 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 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.

CLOSING WINDOW

For 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 a rare chance to try to change public perception.

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

At least one low-polling candidate, former U.S. Representative Beto O’Rourke, is not interested in “launching missiles at any candidates,” because while an attack is the best way of going “viral,” it has no lasting impact, his campaign adviser said.

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

The window is closing, Shrum warned.

“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.

DUELING OVER HEALTHCARE

What the U.S. healthcare system should look like has proven the most divisive topic during the first two debates.

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

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

The most liberal candidates like Warren and Sanders advocate a complete government takeover of health insurance, which would eliminate private insurance and raise taxes.

The more centrist wing, led by Biden and joined by Klobuchar, say a public option should be available but not mandated. Biden argues that the electorate is not interested in a government takeover of health insurance.