How to watch: 8 p.m. to 11 p.m. Eastern on ABC, Univision and on streaming services.

Moderators: The debate will be hosted by George Stephanopoulos, David Muir, Linsey Davis and Jorge Ramos.

Candidates: Former Vice President Joseph R. Biden Jr., Senator Elizabeth Warren, Senator Bernie Sanders, Senator Kamala Harris, Mayor Pete Buttigieg, the entrepreneur Andrew Yang, Senator Cory Booker, former Representative Beto O’Rourke, Senator Amy Klobuchar and former housing secretary Julián Castro.

CreditTamir Kalifa for The New York Times

Below the marquee matchup between Mr. Biden and Ms. Warren, perhaps the most intriguing subplot onstage will be among the candidates stuck in the single-digits in polling trying to position themselves as the leading, less ideological alternative to Mr. Biden.

For now, Ms. Warren and Mr. Sanders seem to dominate the left. Who can emerge as Mr. Biden’s leading rival for the center-left?

The various candidates each have different theories of the case. Mr. Buttigieg has been making a generational argument, though he has increasingly waded into more centrist and unifying grounds. “We need real solutions, not more polarization,” he said in his first TV ad that aired in Iowa.

Ms. Harris, who confronted Mr. Biden directly in the first debate, has tried to position herself as a tough former prosecutor who could take on President Trump one-on-one. Ms. Klobuchar is probably closest to Mr. Biden ideologically and has sold herself as a Midwestern moderate (who would also be a history-making first female president). Mr. Booker has, like Mr. Biden, promised to unify the nation and positioned himself as a healer. But while Mr. Biden has focused chiefly on beating Mr. Trump, Mr. Booker has said from the start that is “a floor not a ceiling” for 2020.

CreditNick Cote for The New York Times

If other candidates are itching to go after Ms. Warren, Mr. Sanders has shown no inclination to squabble with the other leading liberal in the race.

At his debate preparations in Colorado this week, Ms. Sanders has focused on what he has been talking about his entire political career: limiting the power of corporations, installing a single-payer health care system and requiring billionaires to pay more to subsidize a broader social safety net.

That’s not likely to draw him into much of a contrast with Ms. Warren, but it may lead him into a fight with Mr. Biden, either as a tag-team partner with Ms. Warren or on his own.

Mr. Biden, aside from ideological differences, is Mr. Sanders’s chief competitor for Democratic primary voters. Their supporters tend to be lower-income, less educated and far less tuned in to the day-to-day machinations of the presidential campaign than those who back candidates like Ms. Warren or Mr. Buttigieg.

So for Mr. Sanders, a clash with Ms. Warren does less good than showcasing his ideological contrast with Mr. Biden and peeling support away from the former vice president.

Unlike many onstage in Houston, the question for Ms. Harris is not whether she can have a breakout moment. She can. She already has. The question — a harder one to answer — is whether she can turn a strong debate performance into sustained political momentum.

Ms. Harris’s first debate takeover of Mr. Biden of his past work with segregationists and busing led to a quick rise in the polls that quickly faded.

In the second debate, Ms. Harris arrived as the subject of attacks herself — most sharply by Representative Tulsi Gabbard of Hawaii, who will not be onstage this week — and delivered a more uneven performance.

Ms. Harris has herself among the top-tier candidates but that is the kind of phrase often best left for others to utter. Her mandate on Thursday is to show that to be the case, and then have public polling demonstrate the same.

Our colleague Michael Grynbaum wrote today about ABC’s decision not to delay Thursday’s broadcast, leaving censors helpless to bleep any blurted profanities:

Faced with profligate profanities on the campaign trail — and at least one candidate who publicly threatened to work blue on its airwaves (ahem, Beto O’Rourke) — ABC News issued a warning this week to the 10 Democrats appearing on the debate stage in Houston on Thursday: Keep it clean, folks.

“We wanted to take this opportunity to remind you that, as the debate will air on the ABC broadcast network, we are governed by Federal Communications Commission indecency rules,” Rick Klein, the network’s political director, wrote in a memo forwarded to campaigns by the Democratic Party.

“Candidates should therefore avoid cursing or expletives in accordance with federal law,” Mr. Klein added, presumably sighing deeply.

The fact that the debate will be carried on regular broadcast airwaves — instead of cable — means the network could face penalties from federal regulators if obscenities are uttered.

Jonathan Martin and Astead W. Herndon contributed reporting from Houston.

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