CONCORD, N.H. — Sen. Bernie Sanders (I-Vt.), the top vote-getter in the first two presidential primary contests, has staked a fragile claim to being the Democrats’ front-runner.

Whether he stays there, however, hinges on the answer to an existential question: How broadly can a white, male, 78-year-old democratic socialist expand his reach?

What has gotten Sanders this far is the fact that he has the most loyal and fervent supporters within the Democratic coalition. This is no small feat, especially for someone who isn’t actually a member of the party.

He has also been helped by the fact that Democrats, in their desperation to find someone who can beat President Trump in November, find themselves at a moment of extraordinary political fluidity.

According to the New Hampshire exit polls, half of those who cast their ballots in Tuesday’s primary waited until the final days to make up their minds. Most of those late deciders went to former South Bend, Ind., mayor Pete Buttigieg and Sen. Amy Klobuchar (D-Minn.). This late surge dealt a major blow — potentially a fatal one — to the once-formidable candidacies of former vice president Joe Biden and Sen. Elizabeth Warren (D-Mass.).

Sanders voters, on the other hand, stood out for the solidity with which they stood behind him. Among the 32 percent of primary voters who said they had made up their minds before last month, nearly half (46 percent) cast their ballots for him. As in the past, Sanders did especially well among the youngest and most liberal voters.

But in a sprawling field, Sanders has a smaller base, proportionally, than he did when he ran for the nomination in 2016. Four years ago, he basically split the votes with Hillary Clinton in Iowa and trounced her in New Hampshire, topping 60 percent, which was one of the biggest victories ever in a contested Granite State primary.

This year, he won those states with far lower shares of the vote, getting 26.5 percent in Iowa and 25.8 percent in New Hampshire. He finished barely ahead of political newcomer Buttigieg in both states. And Buttigieg maintains a slight lead in the race for convention delegates.

It is possible to imagine Sanders slogging his way to the nomination the way Trump did in 2016, repeatedly winning pluralities against fractured opposition from the party establishment. The potential for that is greater if lower-performing candidates such as Biden and Warren remain in the race.

On the other hand, Sanders’s vaunted online fundraising machine has yet to come up against the even greater resources of former New York mayor Mike Bloomberg, who is blanketing the national airwaves in advance of next month’s Super Tuesday contests. Democratic establishment forces are increasingly viewing Bloomberg as their billionaire backstop against the possibility of running against Trump with a socialist at the top of the ticket.

Sanders is well aware that he must expand his support, including among constituencies that he lost to Clinton in 2016. During his victory speech on Tuesday night, he talked of building an “unprecedented multigenerational, multiracial political movement.”

He has been making an effort to win more Hispanics, for instance, and there are tentative signs that it is paying off. An analysis by UCLA’s Latino Policy & Politics Initiative found that in Iowa’s 32 precincts with the highest density of Hispanic voters, Sanders got 52 percent of the vote — more than triple what Biden and Buttigieg, his closest competitors, did.

Better tests are coming soon. One will be how well Sanders does in the Feb. 22 caucuses in Nevada, a state where nearly one-third of the residents are Hispanic and that he lost to Clinton in 2016. Another will be the Feb. 29 primary in South Carolina, where African Americans dominate the Democratic electorate. Clinton beat him there by a margin of nearly 3 to 1.

Sanders is also going to face more pushback from other constituencies within the party. In Nevada, the hotel and casino industry’s powerful Culinary Union has begun sending out fliers, in both English and Spanish, warning its members that his Medicare-for-all proposal would mean the end of their generous union-negotiated private health benefits.

Sanders is already on the air with television ads in California and Texas, his strategist Jeff Weaver told me, and will soon expand his paid-media campaign into Colorado, North Carolina and other states.

As recently as October, national polls had Sanders running a distant third to Biden and Warren. That he now finds himself in the position of being the man to beat is an astonishing achievement. In the weeks to come, we will discover whether he has transformed the Democratic Party, or merely provided a foil for someone who will.

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