JOHNSTON, Iowa — For weeks undecided Iowans have been saying that they may make their decision based on the last candidate they see in person. On Monday night, that sentiment paid off for Senator Amy Klobuchar of Minnesota at Johnston Middle School.
The senator was a surprise guest at the Johnston 2 precinct, popping in just after the caucus began to make one final pitch to the more than 350 attendees.
“The most important thing is that we win in the general election,” she said, adding that if she was the nominee, the party would rebuild the “blue wall” that President Trump had cracked in Michigan, Pennsylvania and Wisconsin in 2016. “We’ll make Donald Trump pay for it,” she added.
While the final results were unclear, there was no shortage of drama inside the middle school gymnasium, where each of the campaigns staked out room on the yellow bleachers. A handful of undecided Democrats were swarmed by the precinct captains of the most competitive campaigns, who were charged with lobbying people to back their candidate.
Ms. Klobuchar came here because Johnston is a Des Moines suburb that flipped from Republican to Democratic in 2018. It is filled with the sort of moderate Democrats and Republicans she is banking on to support her. Joseph R. Biden Jr., the former vice president, was also counting on support from those same Democrats, but the results of the first alignment were disappointing for him.
“I wish there was a way to tell who is undecided,” said Gregory Davis, a retired Lutheran minister who served as Ms. Klobuchar’s precinct captain.
As caucusgoers trickled in, the most organized campaigns scurried to track who was showing up. Rhonda Gooding and Lara McAdams, the precinct captains for Senator Elizabeth Warren of Massachusetts and Pete Buttigieg, the former mayor of South Bend, Ind., combed through their lists and called supporters who they had not seen.
Over in the section reserved for Biden supporters, the precinct captain Penny Schempp began to get nervous. She had far less than 15 percent of the room, meaning that without an infusion of Biden supporters he would not reach viability — forcing his group to make their second choice.
As Ms. Schempp fretted, a supporter of Andrew Yang, an entrepreneur, came by to lobby her to switch to the rookie candidate.
“I just need to have someone with experience in government,” Ms. Schempp said. The Yang supporter replied that Mr. Biden was responsible for the impeachment inquiry into Mr. Trump.
“He shouldn’t have gotten his son that job for $50,000 per month,” the Yang supporter, who declined to give her name, added. But her argument fell flat.
The first alignment results were indeed bad for Mr. Biden here. With 37 supporters, he fell far short of the 54 caucusgoers required to reach the 15 percent threshold.
Senator Bernie Sanders of Vermont won the first alignment with 76 supporters, followed by Ms. Klobuchar with 75. Mr. Buttigieg had 70 and Ms. Warren 59.
Mr. Yang, who had 33, and the billionaire Tom Steyer and Representative Tulsi Gabbard of Hawaii, who had four each, also failed to reach viability.
Then came the realignment, a frenetic 15-minute period that saw the viable candidates’ supporters swarm like vultures into the sections reserved for Biden and Yang supporters.
But not everybody picked a second choice. About half of the Yang group walked out. Some Biden backers did, too.
Melanie Weatherall, a 50-year-old nurse who wore a blue sequined dress with BIDEN in sparkly letters across the back, refused to realign to another candidate when he was not viable.
“Nobody else can beat Trump,” she said. “They said we should go to Amy, but what is the point? They’re going to mess it up when they can’t beat Trump.”
In the end, Ms. Klobuchar picked up the vast majority of Biden supporters here — adding 31 to her total in the realignment. Her new supporters included Ms. Schempp, the Biden precinct captain, and Greg McCabe, a 76-year-old retiree who said Ms. Klobuchar was his second choice all along.
“I like Amy’s integrity,” he said. “I only backed Biden because I thought he would be better at foreign relations.”
Ms. Klobuchar’s 106 supporters were more than enough to win the precinct. Mr. Sanders, who had the most after the first alignment, placed second with 83, followed by Mr. Buttigieg with 81 and Ms. Warren, who did not win any new supporters in the realignment. She finished with 59.
A few hours later, Ms. Klobuchar appeared at the Des Moines Marriott before supporters. There were no caucus results to be celebrated, or explained, or mourned, given the state party’s problems with reporting the data. Instead, like several other presidential candidates, she couched her Iowa finish as a surprise.
“We know one thing — we are punching above our weight,” Ms. Klobuchar said. “Somehow, some way, I’m going to get on a plane tonight to New Hampshire, and we are bringing this to New Hampshire.”