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Deep down, does Bernie Sanders actually believe he can beat Hillary Rodham Clinton for the 2016 Democratic presidential nomination?

After watching and interviewing him over the last several weeks, it’s clear that the Vermont senator thinks he could win, yet also knows that it will take the political equivalent of lightning in a bottle.

“It’s one thing for people to say that the country is going in the wrong direction, that we need to change directions,” Mr. Sanders told me recently. “But is there the energy to get seriously involved in the campaign? Can the thousands of people coming to our events be turned into a real political organization? Will the people showing up this summer also show up to vote for our message next winter in the primaries?”

People may be surprised to learn that Mr. Sanders isn’t some starry-eyed political fantasist, despite his socialist leanings and visions of moving America closer to a welfare-state system like those of Denmark and Sweden, which he praises as models of fair-minded government on the campaign trail.

After more than 30 years as an elected official, he is shrewd enough to know that the huge crowds he is drawing (including 7,000 people in Portland, Me., on Monday night) won’t mean much if they don’t support his candidacy in the way that matters, by rallying support from friends and acquaintances, and, of course, voting for him when the time comes.

One of the problems for Mr. Sanders is that his political organization is still relatively tiny (a few dozen paid staff members at this point, compared to hundreds for Mrs. Clinton) and can only grow so fast. And it is not growing as fast as the crowds showing up at his campaign events.

Why does that matter? Like another presidential candidate from Vermont, Howard Dean, in 2003, Mr. Sanders is using the summer before the primaries to build political momentum by holding events in reliably liberal enclaves where thousands of voters are flocking to his message, and the news media is spotlighting the big turnout.

But Mr. Sanders doesn’t have big teams of campaign staff on the ground in Portland, Madison, Wis., Minneapolis and elsewhere who can perform labor-intensive follow-up with virtually every voter who attended his rallies. That kind of outreach is a specialty of Mrs. Clinton’s army of organizers and volunteers.

Give us time, say Sanders advisers, who are at this point focusing manpower and money on the two early-voting states, Iowa and New Hampshire.

“We have an incredibly great message, and a messenger that people are excited about,” said Pete D’Alessandro, who is overseeing the Sanders campaign in Iowa. “But the key is the extent to which we can turn a grass-roots campaign into a winning campaign.”

For Mr. Sanders to win the Iowa caucuses, which some Clinton allies and advisers now see as a real possibility, his organization and the savvy, experienced operatives running it will have to scale up substantially.

In the end, of course, there remains the threshold question: Do Americans hunger for radical change intensely enough, and in big enough numbers, to defeat a popular, well-financed Democrat who is widely considered more mainstream and electable than Mr. Sanders?