Should Mr. Biden prevail in South Carolina — an outcome that is no longer seen as a near-certainty — there could be enormous pressure on the other moderates in the race to stand down and give him a clean shot at Mr. Sanders.

Ms. Warren, meanwhile, did not appear to have received a significant bump in Nevada after a debate on Wednesday that was widely seen as her strongest of the campaign. The impact of her dramatic confrontation with the billionaire candidate Michael R. Bloomberg may have been muted here, because so many early votes were cast before it. She now faces the ungainly challenge of seeking to capitalize on the energy of that debate without having triumphed, or even fared especially well, in the contest immediately following it.

At a large rally in Seattle on Saturday, Ms. Warren declared there were “a lot of states to go, and right now I can feel the momentum.” Declining to follow other Democrats in taking aim at Mr. Sanders, she continued deriding Mr. Bloomberg and his self-funded candidacy.

The fragmentation of the vote among the other candidates, not only in Nevada but in the coming primaries, is likely to strengthen Mr. Sanders. After the split decision in Iowa, where he shared the lead with Mr. Buttigieg, and a modest victory in New Hampshire, he appeared to prove his ability to win convincingly in a more diverse state, an outcome that often eluded him in his 2016 bid for the Democratic nomination.

With its mix of Hispanic, African-American and Asian-American voters, Nevada offered Mr. Sanders a rejoinder to critics who claim he cannot broaden his appeal beyond his base of white liberals.

Mr. Sanders’s steady progress in the primary contest has come amid widespread grumbling and occasional howls of alarm from the Democratic establishment, which views Mr. Sanders — a 78-year-old democratic socialist who has never joined the party — and his movement with a combination of fear and distrust. The anxiety deepened this weekend in the aftermath of reports that government intelligence officials believe the Russian government is aiding his candidacy, and after Mr. Sanders acknowledged that he was briefed on the Russians’ apparent intervention a month ago.

Yet his coalition in Nevada — where 35 percent of the voters were not white, according to entrance polls — bodes well for his prospects in the 15 states and territories that will vote on the most important day of the race in just over a week. The Super Tuesday contests include large, diverse states such as California, Colorado and Texas, and the delegate lode is so hefty that if Mr. Sanders performs well, it will be difficult for one of his rivals to catch up given the unflagging dedication of his supporters.