It has become a habit to scold Democratic voters who say electability is their standard in deciding whom to support for their party’s presidential nomination. Forecasts made hours before Election Day three years ago went spectacularly awry, so who can know what will happen in November 2020?

Yet like it or not, the most important watchers of the Democratic debate on Thursday will be electability voters, who happen to constitute a majority of the party. And they are right to believe that the priority in 2020 is defeating President Trump. A man who invents the trajectory of a hurricane is not exactly someone whom we should entrust with four more years of power.

Still, if the question of who can win is a constant, the dynamic going into this encounter is very different from that of July’s face-offs — and not just because 10 candidates who were there before will be missing. Rather quickly, the Democratic presidential race has come down to three candidates, and then everyone else.

The battle for supremacy is between former vice president Joe Biden and Sen. Elizabeth Warren (Mass.), with Sen. Bernie Sanders (Vt.) holding on to his loyalists but having trouble reaching beyond them. The seven others will have to decide which of these three they most need to bring down.

Warren has had by far the best 2019 of anyone. She is still rising in the polls, and she was greeted with warm jubilance at the New Hampshire Democratic State Convention on Saturday. This makes her a target in a way she wasn’t before.

Warren’s supporters have been the sharpest critics of the electability test because it is so often used against her. The doubts about whether she can beat Trump are sometimes expressed in ideological terms (“she’s too left”) and, much more guardedly, about who she is. Can a one-time Harvard professor win Pennsylvania or Wisconsin? Could the sexism that helped undermine Hillary Clinton also undercut Warren?

Bringing up sexism, of course, risks giving sexists power. Is it any different to vote against Warren because she’s a woman, or because other voters might? Either way, the result is the same. When Democratic voters raise the question of sexism, they do so guiltily. But they raise it.

Warren therefore has one big, if amorphous, task: to convince doubters she can beat Trump. How do we know this? A fascinating poll in June by the Democratic data firm Avalanche found Biden ahead with 29 percent to 17 percent for Sanders and 16 percent for Warren. But when the same voters were asked whom they would make president with a “magic wand,” 21 percent picked Warren, with Biden and Sanders getting 19 percent each. The bottom line: If Democrats think she can win next November, she can win primary and caucus contests in February and March.

Warren is fully aware of the burden she must meet. “We need to win in 2020,” she told New Hampshire Democrats. “I get it. There is a lot at stake. But we can’t choose a candidate we don’t believe in because we’re scared.”

It was an indirect but pointed barb at Biden, whose 10-point gap on Avalanche’s preference and “magic wand” questions underscores why he needs a commanding performance this week. A Washington Post-ABC News poll released Sunday brought the point home. Overall, the 76-year-old former vice president was preferred by 29 percent of Democrats to 19 percent for Sanders and 18 percent for Warren. But while 45 percent saw Biden as the candidate having the best chance of defeating Trump, only 24 percent said he would make the best president — to Warren’s 20 percent and Sanders’s 16 percent.

Biden has to reinforce the idea that he is the safest choice against Trump, a view that was brought into doubt by his less-than-stellar outing in the first debate in June and by various missteps and misstatements that critics link to his age. Easing such concerns is his one and only job on Thursday.

For Sanders, who also got an enthusiastic reception in New Hampshire, the question is whether he can break out beyond his seemingly rock-solid base of loyalists. A “New Bernie” is both an impossibility and a bad idea for his brand as a conviction politician. But he won’t win unless he can expand his market share, a phrase I use with apologies to all democratic socialists.

The take I have offered leaves out “everyone else,” which is in one sense unfair. California Sen. Kamala D. Harris and South Bend, Ind., Mayor Pete Buttigieg are within striking distance of the top three. Someone among Thursday’s remaining five could catch a break if one of the current leaders falters.

But that is the point: For now, nearly two-thirds of Democrats support one of the three leaders. And things are likely to stay that way if Sanders’s devoted band keeps the faith — and if Biden and Warren convince Democrats that they can, indeed, throw the world’s most inept meteorologist out of the White House.

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