All of which suggests that the 2020 Democratic field would benefit from more candidates who made an effort to appeal to both the left and the center.

Such a candidate might end up winning the nomination and crushing Trump. Or he or she could sharpen the eventual nominee, by getting that nominee to reach beyond the party’s liberal base.

I expect that the campaign’s next phase will feature several candidates who try to do so. Obama, for example, has advised potential candidates that he “sees a relatively open space for a more moderate Democrat,” as The Times reported. Two Obama favorites who could have filled the role — Mitch Landrieu, the former New Orleans mayor, and Deval Patrick, the former Massachusetts governor — aren’t running.

Joe Biden remains the most obvious possibility. Even without having entered the race, he continues to lead just about every poll of Democratic voters. Beto O’Rourke will probably run too, and he’s shown an intriguing appeal across the left and center. Democrats typically lose Texas by between 10 and 30 percentage points. O’Rourke lost his Senate race by fewer than three points, and his coattails helped elect some House Democrats.

Amy Klobuchar is making a similar play. Her kickoff speech name-checked John F. Kennedy, Reagan, Martin Luther King Jr. and Walt Whitman. Similarly, John Hickenlooper, the former Colorado governor, described himself last week in Iowa as “a doer,” in contrast to the “dreamers and debaters” from Congress. Sherrod Brown, the Ohio senator, also has the potential to win self-identified moderates.