It is unclear if any of these men will enter the race — particularly Mr. Biden, who, associates say, is ambivalent about running after over three decades of presidential fits and starts.

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There is also great uncertainty about lesser-known potential contenders who could be formidable — and if the Trump phenomenon proves anything, it is that making any assumptions about candidates is folly.

Will Senators Sherrod Brown of Ohio and Amy Klobuchar of Minnesota — who both just won third terms in presidential battleground states — go beyond their flirtation with running? And what of the roster of current and former Democratic governors, like Steve Bullock of Montana or Terry McAuliffe of Virginia?

Meeting the moment

Presidential politics recalls the adage that 80 percent of life is showing up.

Except in the case of primaries. Then, it’s about knowing when to show up.

That question, more than anything, is what looms over Ms. Warren’s candidacy: Would she be the president today if she had run in 2016, as some liberal activists and admirers urged her to? Mr. Sanders ended up filling the void on the populist left and ran a surprisingly strong campaign against Hillary Clinton.

Ms. Warren was hardly the first White House hopeful to risk waiting. Bill Clinton, for example, opted out of the 1988 campaign and still became president four years later.

But the more recent history of presidential calculations suggests that candidates are wiser to run when the moment presents itself. That is what Mr. Obama did in 2008 after just four years in the Senate, the same period Ms. Warren would have served by 2016. Some Democrats think 2020 is Mr. O’Rourke’s moment: He has been in the House for just six years, but many liberals see his energy and freshness as inspiring.