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Sanders’ operation tried to seize on opportunities presented by the unlikely sensation that the gruff 74-year-old senator was creating among young voters. Organizers got a list of 25,000 high school seniors to target, aware that anyone who would be 18 or older in November is eligible to caucus.

In the tiny hamlet of Underwood outside Cedar Rapids, the campaign signed up 35 12th graders — potentially enough to tip the balance on caucus night in a town of only 932 people.

All of it was a bet on turnout. By the day before the caucuses, the campaign’s internal models showed that it would take 170,000 or more, second only to the record 240,000 that showed up in 2008, for Sanders to win.

“The thing that keeps me up at night — What if we are wrong? What if everyone’s right, and these people have just been coming to rallies?” D’Alessandro said as he headed into the final weekend.

For most of the early months, Sanders continued to shun many of the trappings of a big-league political campaign. He did not hire a pollster or set up a voter modeling operation until October.

In retrospect, his campaign manager, Weaver, said, “If I had known the money would be there, I would have scaled up bigger earlier.”

There were rough patches that showed how hard it was to go from running for office in a small, ethnically homogeneous state to a national campaign. Sanders seemed to be taken off balance when Black Lives Matter protesters began showing up at his events. And pundits wondered whether he had blundered at the first debate when the biggest controversy surrounding Clinton came up and he declared, “The American people are sick and tired of hearing about your damn emails!”

But the fact that whether he could beat Clinton in Iowa was even a question became an accomplishment in itself, given how far back Sanders had started.

And now, a party that expected a coronation is settling in for a marathon.

Alice Crites contributed to this report.