The first primary was marred by controversy, as technical issues with vote reporting resulted in a three-day delay in vote counting in the Iowa caucus, as well as subsequent recounts. The certified results of the caucus eventually showed Mayor Pete Buttigieg winning the most delegates, while Senator Bernie Sanders won the popular vote in the state. Sanders then won the New Hampshire primary in a narrow victory over Buttigieg before handily winning the Nevada caucus, solidifying Sanders’ status as the front-runner for the nomination.[6][7]

Biden, whose campaign fortunes had suffered from losses in Iowa, New Hampshire and Nevada, made a comeback by overwhelmingly winning the South Carolina primary, motivated by strong support from African American voters, an endorsement from South Carolina U.S. Representative Jim Clyburn, as well as Democratic establishment concerns about nominating Sanders.[8] After Biden won South Carolina, and one day before the Super Tuesday primaries, several candidates dropped out of the race and endorsed Biden in what was viewed as a consolidation of the party’s moderate wing. Prior to the announcement, polling saw Sanders leading with a plurality in most Super Tuesday states.[9] Biden then won 10 out of 15 contests on Super Tuesday, beating back challenges from Sanders, Warren, and former New York City Mayor Michael Bloomberg, solidifying his lead.[9]

https://en.m.wikipedia.org/wiki/2020_Democratic_Party_presidential_primaries