There’s growing concern among Democrats that their July 2020 convention in Milwaukee could open without a candidate who receives a majority of the vote on the first ballot. The last time that happened to the Democrats was 1952.

There are now four candidates—Joe Biden, Pete Buttigieg, Bernie Sanders and Elizabeth Warren—who have enough support and money to be competitive through March. A fifth candidate, Mike Bloomberg, has enough cash to burn a wet mule and a strategy that says he doesn’t care how he does until the 27 primary...