Former Vice President Joe Biden’s status as a front-runner—and the distance that many other Democratic presidential candidates must travel to close the gap with him—come across clearly in the new Wall Street Journal/NBC News poll, which gives an early indication of where enthusiasm rests among the party’s primary voters.

Some 70% of voters who intend to cast ballots in a Democratic primary say they are enthusiastic about Mr. Biden or are comfortable with him. That is a higher share than that received by any other candidate...