HOUSTON — One night only.

For the first time, the 2020 Democratic presidential debate field has been culled to the point that all 10 qualifiers can compete on a single stage on the same night here on Thursday.

That means Democratic voters will get a look at the front-runners in the polls — former Vice President Joe Biden, Massachusetts Sen. Elizabeth Warren and Vermont Sen. Bernie Sanders — in action against one another, and against the tiers of candidates trying to break through before the Iowa caucuses in February.

It is Biden and Warren — first and second in most recent national polling — who avoided each other by the luck of the draw in the first set of Democratic debates, in Miami in June and Detroit in July.

Biden and his aides have hinted strongly in recent days that he may go after Warren, at least indirectly. But she's succeeded in two previous debates by sticking to her policy agenda, and her rollout of a new Social Security plan on Thursday suggests she doesn't want to spend much time talking about herself or the other candidates.

Here are five things to watch for at Thursday night's ABC News-hosted debate at Texas Southern University:

1. Can Biden beat the left?

Last time they were on stage together, Warren and Sanders essentially locked arms and formed a phalanx against more centrist candidates. With Warren making a case for what she calls "big, structural change" and Sanders still calling for a political revolution, Biden has been the chief counterweight to liberalism in the primary contest so far.

But heading into Thursday's debate, he's been losing ground. At his peak in early May, Biden crested at more than 41 percent in the Real Clear Politics average of national polls, with a bigger share of Democratic primary support than the next five candidates combined. Now, with 26.8 percent, Biden has less than the tandem of Sanders (17.3 percent) and Warren (16.8 percent).

It doesn't take a political savant to understand that the threat from his left has grown rather than subsided — or that most of the change has come from Warren gaining. Biden's team has previewed an argument that Warren's vaunted "plans" aren't plausible. He may try to tie Warren to Sanders and argue their dreams are too big to succeed. Whether that's a salient point in the time of President Donald Trump — and whether Biden can sell it — remains to be seen.

2. Warren's message discipline

What Warren wants to communicate to voters is simple, according to an aide: how she plans to fix what she sees as broken in America. That's not about the other candidates or even about Trump — except to the extent that his policies have contributed, in her telling, to the state of dysfunction.

But as the candidate who has ascended from relatively negligible support at the start of the race to the top tier, she can expect to be the target of attacks from her rivals Thursday night. That figures to test her ability to keep her focus on policies and not personalities — and on what she thinks are the right remedies to help voters.

If Warren gets dragged into a lot of back-and-forth on her own record — or even the details of her vision rather than the broader messaging that has worked for her so far — that's territory she doesn't want to be in.

3. Sanders first

In 2016, Sanders had the anti-establishment vote all to himself. But this time around, his coalition has fractured among other candidates. For the Democrats who want revolutionary change, he's struggled to differentiate himself enough from Warren to prevent her rise. And for Democrats who are wary of nominating a woman in the shadow of Hillary Clinton's 2016 defeat, he hasn't given them enough of a reason to pick him over Biden.

This debate gives Sanders a chance to carve a path for himself that pulls voters from both camps toward him and away from the other front-runners. But he'll have to draw strong contrasts to do that. History suggests a frontal assault on rival Democrats, but his best play might be showing that he can take a democratic socialist fight to Trump as well as, or better than, Warren can with her liberal capitalist approach or Biden can with his center-left platform.

Watch for him to go after The Donald.

4. Owning the stage

Sen. Kamala Harris of California, currently fourth in the polls, has had big moments — good and bad — in her previous two debates. She's shown she can play on the big stage and command attention. But what she hasn't demonstrated is consistency, either in the debates or on the campaign trail.

Democratic voters will be looking to see whether she's able to turn in a solid performance from start to finish that includes some of the flashes of brilliance they like on political questions but more of an ability to sustain her presence on substance.

Mayor Pete Buttigieg of South Bend, Indiana, faded into the background in his first two debate performances. Skilled at the art of creating big moments for himself when he's the only candidate at a town hall or on the campaign trail, he's struggled for oxygen when matched against rivals. Can he step up and show he's a boss rather than the young guy who gets plaudits for winning a seat at the adult table and behaving himself?

5. Go big or go home

For the candidates on the edges of the stage — those polling worst and in danger of being knocked out of the race sooner rather than later — it's time to go big or go home. And it's probably not enough to just land a punch or two on one of the front-runners.

There's a lot of risk in gimmicks, and it's likely one or two will implode with a grand last gasp. But it's possible for one candidate to turn in the performance of his or her life — with a new frame and new policy wrinkle or two — and earn a second look from voters.

There's not much reason to hold back now.