Like so many others, Givan Tichy was torn between Hillary Clinton and Bernie Sanders at the start of caucus night. But after a long night of debating, first time caucus-goer Tichy decided to go with his heart and feel the 'Bern.' (Alice Li/The Washington Post)

It was the first big strategy session between Hillary Clinton and the yet-to-be-announced manager of her yet-to-be-launched presidential campaign.

As they huddled that day last March in the converted barn that serves as her home office in Chappaqua, N.Y., Robby Mook thought he ought to call the presumed Democratic front-runner’s attention to a rumor that was starting to go around.

“Bernie Sanders is going to be a real contender if he gets in,” Mook recalls warning his new boss. Mook grew up in Vermont and knew what happened to those who underestimated Burlington’s onetime mayor.

“I’ve seen this guy in action,” he said.

Clinton, he said, had the same uneasy feeling about her ex-colleague from the Senate.

Sanders turned out to be more dire a threat than either could have anticipated.

Not only did Sanders live up to Mook’s assessment of his political skills, but in Iowa, he fit the moment and the passions of a Democratic base that put him in position to pull off one of the biggest upsets in the history of the caucuses.

Clinton was determined not to repeat the mistakes of 2008, when her seeming inevitability melted in Iowa like a snowman in April.

This time, she accepted the state and its quirky caucus system on their own terms, building a formidable army of operatives and volunteers. Staffers at the Brooklyn headquarters were instructed to cater to the needs of state-level organizers.

The meticulous work proved crucial as Clinton’s wide lead began to evaporate over the summer. She had to grapple with a Republican onslaught over her handling of the 2012 attacks in Benghazi, Libya. And she endured the controversy over her use of a private email server.

In Sanders, Clinton came up against a challenger who lacked her organizational firepower but campaigned as a repudiation of the front-runner’s establishment credentials. Sanders’s vow to blow up the big banks effectively painted Clinton as an ally of Wall Street.

The mood in Iowa proved unwelcoming to another establishment candidate, former Maryland governor Martin O’Malley. His record of achievements checked just about every box on the liberal wish list but never found traction. He quickly fizzled despite early expectations that he would have his moment, too.

Sanders’s rise in Iowa exposed weaknesses that could haunt the Democratic front-runner.

A self-described democratic socialist who was running 30 points behind her in the polls as recently as November, Sanders drew support from young people, liberals and independents.

The photo finish showed that Republicans are not the only voters looking for qualities beyond experience and electability.

Building on a message

The reality of what was happening dawned on Sanders when he got stuck in traffic on a Sunday morning.

The senator from Vermont had launched his long-shot bid five days before. He was headed to a May 31 rally at the American Indian Center in Minneapolis, his first big campaign event outside his New England home turf.

But Sanders was still blocks away — and the car he was in was not moving.

“Is there a wreck ahead?” Sanders anxiously asked his field director, Phil Fiermonte.

“No,” Fiermonte replied, “they’re here to see you.”

More than 3,000 of them, many standing outside because the hall was full.

“It never occurred to me in a million years that line was for us,” Sanders recalled in a telephone interview Sunday, as his campaign bus chugged between Marshalltown and Ames on the eve of the Iowa caucuses. “I said, ‘Whoa.’ That was the first inkling that I had that this campaign was catching on.”

And the crowds kept growing — in Denver, in Madison, Wis., even red-state bastions such as Dallas and Houston. At the Phoenix Convention Center in mid-July, he drew upward of 11,000, more than double the number who had shown up there for Republican Donald Trump the week before. The biggest was in Portland, Ore., where 28,000 came to hear him in August.

Still, Sanders barely registered in the national polls. His organization was starting from nothing. Clinton had hired most of the name-brand political talent and had all the big endorsements. Many of those who hadn’t joined her were leery of crossing her.

In the early days of the Sanders campaign, “there was a guy. There was a message. That was it,” said political consultant Tad Devine, who had worked for a couple of Sanders’s congressional campaigns and signed on for his presidential one.

But Sanders had been thinking about doing this for a long time. He had been traveling the country venting his outrage over income inequality, the excesses of Wall Street and the corruption of the campaign finance system. His speeches found receptive audiences, but he did not seem to be resonating beyond them.

In March 2014, he sought out Bill Press, a liberal radio talk-show host who had been chairman of the California Democratic Party in the mid-1990s.

“He told me that he had come to the realization that the only way people take you seriously and listen to your ideas is if you run for president,” Press recalled. “He thought that was kind of silly, but that’s the way it was.”

For his own part, Press said: “I didn’t want to see a coronation. I thought the party would fall asleep, and Hillary is at her best with a challenge.”

Press put together two dinners for Sanders with about a dozen people at his house on Capitol Hill. One was in April, the other in November.

Among those who attended one or both: Susan McCue, a former chief of staff for Senate Minority Leader Harry M. Reid (D-Nev.); Rep. Keith Ellison (D-Minn.); Brad Woodhouse, a former Democratic National Committee spokesman who now heads the liberal super PAC American Bridge 21st Century; Rep. Barbara Lee (D-Calif.); and Alyssa Mastromonaco, former deputy chief of staff in the Obama White House who had answered phones in Sanders’s congressional office back when she was a college student in Vermont.

At the first dinner, “Bernie gave his ideas. We all kicked it around,” Press said. “The main thing that came out of the first meeting was there was a possibility and a need for somebody to raise those issues, but if he ran, he should definitely run as a Democrat.”

Devine led the discussion at the second dinner, and “basically laid out a plan of how to get from here to there,” Press said. “It was much more focused and much more real. Bernie took that, and off he goes. It was only a matter of when, not if, he announced.”

That this would be an unconventional, no-nonsense operation was clear from the outset.

Sanders made his April 30 announcement at a 10-minute news conference on a grassy spot near the Capitol parking lot known as the Senate Swamp. With tourists and congressional staffers wandering in the background, he opened: “We don’t have an endless amount of time. I’ve gotta get back.”

A month later would come the camera-ready formal launch. Sanders — standing against glittering, sun-washed Lake Champlain, and with his hair combed this time — promised a cheering crowd of 5,000: “Today, with your support and the support of millions of people throughout our country, we begin a political revolution to transform our country.”

Weathering the crises

At Clinton’s Brooklyn headquarters, Mook had scribbled a message on a whiteboard: “What have you done for Iowa today?”

Occasionally, someone would erase it, and Mook would write it again.

The line was a reminder of how different the campaign operation was from 2008, when Clinton’s team had debated internally whether to even compete in Iowa.

Nationally, however, the opening months of her presidential campaign were a deluge of bad news for Clinton.

First came the revelation that she had been using a private email account, rather than a government one, for conducting business as secretary of state. Then a spate of stories about the finances of the Clinton Foundation and her six-figure speaking fees. Then came the news that Vice President Biden was considering a late entry into the race — in part because some Democrats worried that Clinton was starting to look like a weak general-election candidate.

In October, it was coming to a head, with the added tests of the first Democratic debate and her grilling before the House Select Committee on Benghazi.

The day before she was to testify, Clinton and her aides were holding a prep session at her dining room table in Washington when communications direction Jennifer Palmieri got a text message telling her to turn on CNN.

Biden was heading for the Rose Garden, with President Obama in tow. That choice of venue could mean only one thing, they knew: Biden was not running.

One crisis was averted. But another one was brewing. While the Benghazi hearing was a chance for Clinton to shine while fending off attacks from political enemies, aides knew that any mistake or gaffe could prompt more doubts about her strength.

The next morning, as Clinton was preparing to leave home for Capitol Hill, she got a jarring call from longtime aide Cheryl Mills. With just an hour to go before the start of the hearing, the Republicans had just dropped a binder full of hundreds of documents they intended to use in their questioning — documents that Clinton had not prepared for.

Among them, Clinton aides say, was an email she had sent to her daughter, Chelsea, on the night of the attacks in which the secretary blamed “an Al Q[a]eda-like group.” GOP lawmakers saw it as a smoking gun, evidence that the Obama administration knew the attack had been terrorism even though officials initially said the incident had been sparked by an anti-Muslim video.

But Clinton lawyer David Kendall had negotiated to prevent something that could have been more damaging. They feared that committee Republicans would have Clinton raise her right hand and be sworn in at the opening of the hearing, which would have produced an image that would be a GOP admaker’s dream. Instead, she signed an oath before the hearing started.

A spokesman for the House committee said the leadership offered Clinton the chance to be sworn in privately to avoid a political spectacle. In addition, said the spokesman, Jamal Ware, the committee does not typically give witnesses advance information but provided the additional documents to Clinton as a result of negotiations with Kendall. Because Clinton had decided which emails to disclose, Ware said, her surprise reflected a “failure on the Clinton team’s part to prepare her with the materials they knew they had turned over to the committee.”

Clinton emerged from October with her poll numbers on an upward swing. Meanwhile, Mook was preparing for the long game. He had already moved organizers into Colorado and Minnesota, which like Iowa are caucus states, set to hold their contests on March 1. In other words, they were just the kind of place where a political movement like the one that Sanders had ignited could run up the delegate score.

As voting day in Iowa drew near, former president Bill Clinton hit the trail for his wife, calling Mook frequently to ask him questions and advice. He urged the campaign to focus more on her biography — which it has.

“Have we told people about how she reformed the educational system in Arkansas?” Bill Clinton would say, according to Mook. “Have we told people about the pre-K work she did? Have we told people about the Children’s Defense Fund?”

Mook added of the former president: “He’s also a junkie. He just likes to talk shop.”

‘The surprise campaign’

Sanders recruited a trusted former aide, Jeff Weaver, to be his campaign manager.

Weaver had left as Sanders’s chief of staff to open a comic book and gaming store in 2009. He had been reluctant to be drawn back into politics, especially for a quixotic venture, but Sanders convinced him it was not. “It became clear to me from the beginning that he was running to win,” Weaver said. “This was not just a speaking tour.”

Sanders also made clear what kind of organization he wanted.

“It had to be a modern campaign, with all the modern tools of running, without becoming a corporatized environment,” Weaver said.

The money came quickly. Devine had told Sanders that he had to raise at least $40 million — and preferably $50 million — if he wanted to be competitive by the Iowa caucuses. By the end of last year, they had raised $73 million. In January, an additional $20 million poured in, almost all of it from contributions that averaged about $27.

That was enough for Sanders to put an additional 139 organizers in Iowa in January, more than doubling the total the campaign was saying publicly. As the political world focused throughout the summer on the gains Sanders was making against Clinton in New Hampshire, his campaign was building an Iowa operation earlier, and with more resources, than was generally recognized

Weaver called it “the other campaign . . . the surprise campaign.”

Sanders’s first Iowa rally, which was in late May in Davenport, drew 700 people — the largest Iowa crowd any candidate had seen at that point. Volunteers were stationed there an hour and a half early to collect information about the attendees.

Typically at that early stage, voters are just starting to shop around for a candidate. But to the surprise of the embryonic Sanders campaign, one-third of those who attended that rally arrived ready to sign up as committed to the candidate.

‘There was this organic thing happening,” said Pete D’Alessandro, the Iowa director. “That’s where Senator Sanders knew it from the beginning. His message — this was something people were waiting to rally behind.”

Although there was no doubt about the enthusiasm, marshalling it into 1,681 precinct caucuses presented a challenge.

The campaign estimated that as many as 40 percent of the supporters it identified had never been to a caucus before. At meetings of his precinct captains, D’Alessandro would ask which of them would be attending their first; sometimes, half the people in the room would raise their hands. And then there was the occasional volunteer who would tell him: “Yeah, I haven’t done this since McGovern,” referring to the Democrat who ran in 1972.

The campaign made an intense effort to train organizers and volunteers on how to canvass and phone-bank. Staffers sent out regular math exercises for them to do over the instant-messaging system Slack, to make sure they understood the calculations they would have to make under the arcane rules on caucus night.

Sanders’s 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 Council Bluffs, 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.

Editor’s note: This story has been updated to include comment from the House Benghazi committee spokesman.