The moderate wing of the Democratic Party swiftly consolidated around Joe Biden’s candidacy ahead of Super Tuesday, as a trio of the former vice president’s competitors, Pete Buttigieg, Amy Klobuchar and Beto O’Rourke, joined forces to endorse him Monday.

The newly scrambled field put Mr. Biden on a collision course with former New York City Mayor Michael Bloomberg—whose campaign confirmed Monday he will stay in the race—for the first time. The two rivals are pitching themselves as the candidate most capable of blocking Vermont...