HOUSTON (REUTERS) - Leading Democratic White House contenders Joe Biden and Elizabeth Warren will share the stage for the first time on Thursday (Sept 12) 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 fund raising 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.

1. BIDEN V WARREN

The condensed debate stage will generate some first-time match-ups, but arguably the most anticipated is the one between former vice-president Biden, the leader in most opinion polls, and 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 Mr Trump in the November 2020 election?

Ms Warren's recent gain on Senator Bernie Sanders of Vermont in polls has also raised speculation that the two friends and progressive allies could abandon their agreement to not criticise each other.

"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 Mr Joel Payne, a Democratic strategist who worked for Mrs Hillary Clinton in 2016.

For 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.

Mr 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 Mr Bob Shrum, a political science professor at the University of Southern California and former Democratic strategist.

2. HOURS WITHOUT A BIDEN GAFFE?

Mr Biden enters the his third debate atop the field, even after his first two debate performances were criticised as lacklustre, while his campaign has had to fend off questions about repeated mis-statements on the campaign trail.

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

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

Mr 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 Mr Sanders, 78, the oldest candidate in the field.

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

"He never won any debate but he managed to stay above the fray. Mr Biden is likely hoping that he can ride a similar strategy to get through a splintered Dem primary."

3. WHAT DOES KAMALA HARRIS STAND FOR?

The candidate who has benefited the most from the debates so far is Ms 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 Ms Harris and Mr Buttigieg were tied with 4 per cent support of Democrats and independents - a big drop for the senator, who once registered at 10 per cent.

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," Mr Shrum said.

4. CLOSING WINDOW FOR BREAKTHROUGHS

For the candidates in the bottom half, such as former Housing Secretary Julian Castro, 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 Ms Harris effectively attacked Mr 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 Ms Warren and Mr Sanders, they will struggle to gain momentum unless Mr Biden declines, experts say.

The window is closing, Mr Shrum warns.

"For those folks to break through, Mr 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," Mr Shrum said.

5. DUEL 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 per cent of people, did not have health insurance in 2018, an increase of almost two million from the year before when 7.9 per cent of people lacked coverage, the Census Bureau report found.

The most liberal candidates like Ms Warren and Mr 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 Mr Biden and joined by Ms Klobuchar, say that a public option should be available to whoever would like to have it but should not be mandated. Mr Biden argues that the national electorate is not interested in a government takeover of health insurance.