DES MOINES — The Democratic presidential candidates pleaded with voters in Iowa for their last-minute consideration on Sunday, competing with the Super Bowl for caucusgoers’ attention and straining against an atmosphere of unusual uncertainty and indecision among Democrats ahead of the first-in-the-nation nominating contest.
The cancellation on Saturday night of a final pre-caucus poll from The Des Moines Register and CNN, because of a survey error, frustrated campaigns that had come to depend on the poll as a reliable omen of caucus results. But strategists for several campaigns said there was a deepening sense that Senator Bernie Sanders of Vermont had an advantage ahead of Monday’s contest.
Public polling has shown Mr. Sanders gaining ground, and he has outspent all of the other leading Democrats on television by a wide margin in recent weeks. A New York Times polling average found Mr. Sanders and former Vice President Joseph R. Biden Jr. tied for first place in the state, with each of them collecting support from about 22 percent of likely caucusgoers. Trailing them in third and fourth place were former Mayor Pete Buttigieg of South Bend, Ind., and Senator Elizabeth Warren of Massachusetts.
Mr. Sanders campaigned on Sunday in a confident tone, concluding a speech in Iowa City by standing beside his wife, Jane, and urging the crowd to imagine them in the White House: “Some of you are unhappy with me and some of you think I might not be a great president,” he said. “Understand, she will be a great first lady!”
Should Mr. Sanders emerge as a convincing victor from Monday’s caucuses, he would aim to carry that momentum forward into next week’s primary in New Hampshire, where he was already seen as having an upper hand, and the Nevada caucuses later this month. But there is still widespread concern among Democratic Party leaders and center-left primary voters about the implications of nominating a self-described democratic socialist to take on President Trump. Mr. Sanders’s allies acknowledge there is little chance he will be able to lock up the nomination without a long fight.
His chief opponents are unlikely to give way easily: Even if he is defeated here, Mr. Biden has a strong national following among moderate voters and African-Americans, while Ms. Warren retains a sizable base among educated liberals and women. And Michael R. Bloomberg, the former mayor of New York City, is looming as a competitor in the March primary states.
Mr. Biden, who has long anchored his campaign in appeals to electoral prudence, tried on Sunday to remind Democrats that the G.O.P. sees him as a strong general-election candidate. He pointed to a morning appearance on CNN by Senator Joni Ernst, the Iowa Republican, in which Ms. Ernst again suggested that Iowa voters might draw unfavorable conclusions about Mr. Biden as a result of Republican attacks on him during Mr. Trump’s impeachment trial. Iowans, Ms. Ernst said, were “very smart.”
“Why don’t you let Joni Ernst know just how smart you are, and caucus for me?” Mr. Biden said, earning a standing ovation from voters in Dubuque.
Calling the impeachment process a “sham trial,” Mr. Biden encouraged Iowans to “hold Trump accountable” by supporting the former vice president on Monday.
But in an outlandish episode that seemed to reflect some senior Democrats’ alarm about Mr. Sanders’s strength here, NBC reported on Sunday afternoon that John F. Kerry, the former secretary of state, had been overheard in a Des Moines hotel talking about the possibility of entering the presidential race to stop Mr. Sanders from “taking down the Democratic Party.”
Mr. Kerry, who has been campaigning across the state as a surrogate for Mr. Biden, wrote on Twitter that he was “absolutely not running for president” and reaffirmed his support for Mr. Biden. But he notably did not deny having had the conversation NBC reported.
If Democratic voters had been anxious, even tortured, about the task of choosing just one option in the 2020 race, over the weekend there was visible enthusiasm for many of the candidates, who were addressing swollen audiences on college campuses and in local gymnasiums.
“The bad news is, there’s no more room inside,” Ms. Warren joked to an overflow crowd at Simpson College. “The good news is, there’s no more room inside!”
Mr. Buttigieg, who addressed a crowd of about 2,000 people on the South Side of Des Moines on Sunday, acknowledged to reporters that his campaign had to clear a high bar on Monday. Among the top candidates in Iowa, Mr. Buttigieg has by far the most ground to make up in national polls and he is counting on a burst of new energy.
“Let’s face it, we need a very strong finish here,” he told reporters in Coralville. “This is our chance to show-versus-tell that we’re building the organization that can turn people out and go on to defeat Donald Trump.”
The senators in the mix repeatedly indicated to voters that they had felt constrained in their campaign activities by an impeachment process that kept them tethered to Washington. Ms. Warren skipped her signature photo lines, explaining regretfully that she needed to “get around to as much of Iowa as possible.” Her internet-famous dog, Bailey, was stepping in to be photographed in her stead.
Senator Amy Klobuchar of Minnesota, who has been contesting Iowa as a vigorous underdog, implored voters here to “have my back” because she had to return to Washington briefly to attend to impeachment duties on Monday. “I wish I could be here at the very last moment, but I can’t,” she told voters in Cedar Rapids.
Ms. Klobuchar, Ms. Warren and Mr. Sanders were all planning to make hit-and-run appearances in Washington on Monday before returning to Iowa for the night of the caucuses.
While a hectic scramble was underway in Iowa, a separate political feud, with potential implications for the Democratic primary, was unfolding on television and social media — between Mr. Trump and Mr. Bloomberg.
Mr. Bloomberg, who is skipping Iowa and using his multibillion-dollar fortune to saturate later-voting states with advertising, has been largely outside the political foreground in the run-up to the caucuses. But Mr. Trump changed that with an overnight diatribe on Twitter, belittling Mr. Bloomberg’s modest height and accusing the Democratic establishment of favoring him over Mr. Sanders.
Mr. Bloomberg, whose lavishly financed campaign was also running an ad during the Super Bowl, responded in a tart fashion that delighted some liberals. His spokeswoman, Julie Wood, said in a statement that Mr. Trump was “a pathological liar who lies about everything: his fake hair, his obesity and his spray-on tan.”
If the exchange was less than dignified, it was also a reminder of Mr. Bloomberg’s looming presence in the race and the possibility that he could wind up as a last-man-standing option for centrist Democrats if Mr. Sanders dominated the early primaries and caucuses this month.
Reporting was contributed by Sydney Ember from Iowa City, Reid J. Epstein from Coralville, Iowa, Shane Goldmacher from Indianola, and Katie Glueck from Dubuque.