Faiz Shakir, the campaign manager for Mr. Sanders, said: “This is a terrific lineup because there will be a real debate over the key set of choices in this Democratic primary.”

The stakes are especially high for candidates like Senators Amy Klobuchar of Minnesota and Cory Booker of New Jersey and former Representative Beto O’Rourke of Texas, who all entered the race with political promise but have struggled to catch fire with voters. The three will vie for airtime in the first debate against candidates who have little momentum, like Mayor Bill de Blasio of New York, as well as against Ms. Warren, who has edged ahead of her rivals in part because of the policy substance of her campaign.

Yet Ms. Warren faces challenges too. Her placement offers a larger share of the spotlight, a chance to soak up extra time on the biggest night yet of the campaign. But it also means she will not get a chance to contrast herself with her top rivals — with, say, Mr. Biden and the credit card industry — and that if anyone on the first night wants to punch up at a top-tier candidate, they will be taking aim at her.

Mr. Buttigieg, who has climbed out of obscurity and risen in the polls, will have a national stage to showcase his generational change argument while standing aside Mr. Biden and Mr. Sanders — who are each nearly 40 years older than he is.

Ms. Harris, who will face off against those three men and six other candidates, has an opportunity to present her contrasting vision of electability based on a multiracial coalition against Mr. Biden’s argument that he can win back white male Rust Belt voters who cast ballots for Mr. Trump.

The selection of the candidate lineups on Friday unfolded like a scene from “The Apprentice,” the former NBC reality show hosted by the man who is now the president. Representatives from the campaigns gathered into an 11th floor conference room at the network’s Rockefeller Center headquarters. Arrayed on a table were two boxes — wrapped in white paper with gold dots on it — labeled “2% and above” and “below 2%” to correspond to the candidates’ polling status.

Each candidate’s name was written on a piece of paper, folded in half, and placed in the appropriate box. The names were drawn from the boxes one by one and affixed onto one of two easels with tape.