If Ms. Warren is the progressive of high-minded policy, Bernie Sanders is the progressive of belligerent revolution. Much of his appeal, dating back to his 2016 presidential run, lies in his grouchy, I’m-not-here-to-make-friends pugilism. For voters spoiling for a fight — whether against Mr. Trump, the Democratic establishment, the 1-percenters — Mr. Sanders has become a cult hero.

But many Democrats fear that the democratic socialist and his rowdy revolutionaries would usher in a second Trump term. This is especially true of moderates, who see Mr. Sanders as so far outside the mainstream that he would drag down other Democrats on the ballot. For much of this race, Mr. Sanders languished in Ms. Warren’s shadow, dismissed as too divisive to be the standard-bearer for a nervous party to which he does not even belong.

But as Ms. Warren has faded, Mr. Sanders has consolidated progressives behind him, while moderates remain split between multiple contenders. After a narrow loss in Iowa to Pete Buttigieg (at least at last count), Mr. Sanders scored a solid win in New Hampshire, which borders his home state. The victory was far from overwhelming, edging Mr. Buttigieg by less than two percentage points. But it was enough to make him the presumptive front-runner.

If he is to lead the entire Democratic Party, Mr. Sanders needs to find a way to reach beyond his core supporters and reassure the moderates who fear he would be a disaster in the general election.

This is no small challenge, in part because of his passionate — some might say rabid — following. The so-called Bernie Bros have a nasty reputation on social media for abusing those who dare to criticize their guy, spewing invective and issuing threats against offending organizations and individuals.

A fight along these lines is brewing in Nevada, whose Feb. 22 caucuses are next on the primary calendar. Organized labor is a powerful force in the state, thanks to the legions of workers who keep the casinos and hotels humming. Recently, the state’s Culinary Workers Union issued a flier outlining the candidates’ positions on top issues. Under health care, they noted that Mr. Sanders’s plan would “end” the union’s hard-won benefits.

This did not sit well with the Bernie Bros, who, the union reports, “viciously attacked” its members with ugly emails, calls and tweets. The union issued a statement condemning the behavior. Bernie fans turned up the heat, blaming union bosses, the media, political elites and the rest of Democratic field for the controversy.