Mr. Biden, whose campaign had been on the verge of collapse a few days ago, ran up huge margins among older voters and those from suburbs and black communities. That support helped him win at least eight of the 14 states that were in play, including a stunning upset in Massachusetts.

The Associated Press declared Senator Bernie Sanders — who won Colorado, Utah and his home state of Vermont — the winner in California, which has 415 of the night’s 1,357 delegates. Mr. Sanders continued to show strength with liberals, young people and the Latino community.

The latest: We’re covering the results in real time and updating our live forecasts.

Go deeper: The Sanders campaign was caught off guard by Mr. Biden’s recent surge, but sees the moderate former vice president as the perfect foil for the promise of a political revolution.

Voices: Many Super Tuesday voters expressed lingering doubts about the Democratic primary field — including, in some cases, the candidates for which they cast ballots.

Michael Bloomberg: The billionaire and former New York City mayor was on the ballot for the first time on Tuesday. His only victory was in American Samoa.