Two of his top rivals, Sens. Bernie Sanders and Elizabeth Warren, have started feuding, raising the prospect of a splintered progressive vote. Pete Buttigieg, a well-funded, well-organized, moderate alternative to Biden, has still not demonstrated that he can appeal to people of color. And three of the five top-polling candidates in Iowa — Sanders, Warren and Sen. Amy Klobuchar — are about to be pulled away from the campaign at the most inopportune time, stuck in Washington for President Donald Trump’s impeachment trial.

Until recently, Democrats had operated on a seemingly universal consensus that Biden — an aging moderate with a record of losing presidential races — would wither in a competitive field. Many of his rivals stitched that thinking into their own plans.

Yet, less than three weeks before the Iowa caucuses, the opposite is turning out to be true. Even critics of Biden’s campaign here have been surprised at his resilience.

“I thought he would be in trouble by now,” said Tom Courtney, a former Iowa state senator and now co-chairman of the Des Moines County Democrats in Iowa.

Courtney, who is neutral in the contest, described Biden’s field operation as “terrible,” with a far lighter footprint in the state than some of his competitors.

Still, Courtney said, “I can read the polls, too. … There’s every chance that he’ll win Iowa.”

Democratic Party officials and operatives working with several of Biden’s opponents now say they no longer believe Biden's support is as fragile as they once believed it was. Instead, they now see a durable floor of support for him at least in the first caucus state.

“I thought he would be in trouble by now." Tom Courtney, co-chairman of the Des Moines County Democrats in Iowa

His opponents are preparing for defections to Biden on caucus night from supporters of more moderate candidates who have dropped out already, or who fail to meet the 15 percent threshold necessary to win delegates. Rival campaigns are urgently working to persuade caucus-goers and potential endorsers in Iowa to shift their support elsewhere.

Earlier this week, Buttigieg was phoning supporters of Sen. Cory Booker in Iowa shortly after his withdrawal from the contest, nurturing potentially critical lines of communication with caucus-goers suddenly without a candidate, according to a source familiar with his outreach.

Courtney, like other Democrats, is aware that there is also a real chance there is no clear winner — with little air between Biden and three other frontrunners. And some Democratic operatives believe that if Biden finishes third or fourth in Iowa and New Hampshire, his support may begin to crumble in later states, including South Carolina, where he now holds an enormous lead.

But no one is betting on an implosion, anymore.

A Monmouth University poll of likely Iowa caucus-goers this week put Biden in first in Iowa at 24 percent. His frontrunner status nationally hasn’t changed, and the burgeoning hostilities between supporters of Warren and Sanders have unnerved many progressive Democrats who fear the distraction could help Biden.

“When progressives fight each other, the establishment wins,” Charles Chamberlain, chairman of the political action committee Democracy for America, said in a prepared statement on Thursday, after audio surfaced confirming a post-debate confrontation between Warren and Sanders over Warren’s accusation that Sanders told her privately in 2018 that a woman could not win the election. “We saw it in 2004 when progressives took each other out and John Kerry slipped through to win Iowa and then went on to lose in November to a very unpopular Republican incumbent. We’re determined to not let that happen again.”

Launching what they called a “Progressives Unite 2020” campaign, DFA and 17 other groups pledged to “focus our fight for the nomination against candidates supported by the corporate wing, instead of fighting each other.”

For Biden, the anxiety on the left represents a turnaround from just last fall, when moderate Democrats were loudly voicing concerns about their candidates and two potential alternatives, Deval Patrick and Michael Bloomberg, announced late runs.

It was only after that unrest, a strategist working with another presidential candidate said, that Biden seemed to “get it together.”

“I can’t tell if he was scared straight or if it was just the longer ramp-up of a more seasoned candidate,” the strategist said. “But it sure seems like ever since that mortal threat of Bloomberg and Patrick, he’s cruising — he’s finding his stride.”

Biden gained endorsements from the ranks of the Kamala Harris, Beto O’Rourke, Julián Castro and Booker campaigns. And he stepped up his fundraising — collecting about $23 million in the final quarter of 2019. Though that was not the biggest haul among the field, in a campaign defined by momentum, it marked Biden’s best fundraising quarter of the year.

Then, early this month, Trump’s intervention in Iran turned the focus of the primary for the first time to foreign affairs, which accentuated long-held policy differences between Biden and Sanders and appeared to elevate each of them with moderate Democrats and progressives, respectively.

While calling the race “still wide open,” former New Mexico Gov. Bill Richardson said that for Biden, “I just think the stars are aligning his way.”

“Now what he needs to do, I believe, is end up either second or third in both Iowa and New Hampshire,” said Richardson, who ran for president in 2008. “And then, I think if that happens, he will have a clear path, because I know he’s strong in South Carolina, and Nevada, I think, will be the state that starts tilting in his direction.”