CORALVILLE, Iowa — Making last-minute pleas to an electorate that has remained widely undecided, the presidential candidates powered toward the end of a year-long Iowa caucuses campaign by focusing Sunday on vows of electability and Democratic Party unity even as they offered sharply different visions of what that meant.

On the final full day of campaigning here before Monday night’s precinct caucuses, crowds overflowed school gymnasiums and campaign offices around the state as the candidates kept a brisk pace to make their final appeals. But it was also a day marked by upheaval and signs of discomfort among top party officials over whether Iowa’s vote will push the presidential race too far to the left.

On Sunday, even as Joe Biden’s campaign aides worked to play down expectations and allies sought to cast doubt on rival Sen. Bernie Sanders (Vt.), an NBC News reporter overheard one of the former vice president’s top surrogates, John F. Kerry, in a hotel lobby speculating over the idea that he would jump into the race.

While the 2004 nominee later denied that he would run in 2020 — “I am absolutely not running for President,” the former secretary of state wrote on Twitter — he did not directly deny the report that he had speculated about it, with NBC saying he cited as a motivating factor “the possibility of Bernie Sanders taking down the Democratic Party — down whole.”

It was, in some ways, a fitting cap to an unusual campaign in which Democrats, after the long campaign here, continue to grapple with which of their many options offers the best chance of defeating President Trump.

The top candidates offered conflicting views of how they would position the party to take on Trump. Biden and former South Bend, Ind., mayor Pete Buttigieg put forward a more centrist view, Sanders vowed to reshape the party in his liberal image, and Sen. Elizabeth Warren (Mass.) presented herself as a unity candidate residing in the middle of those two camps.

As they made closing arguments to Iowa’s voters, Biden and Buttigieg touted their ability to win voters in Republican-leaning districts as a sign they would have broader appeal in the general election. Buttigieg underscored that by citing at one of his last events his favorite Beatles song: “Come Together.”

Sanders, whose brand of politics can border on uncompromising, made an overt appeal on Sunday to those who disagree with him and focused on the most unifying force coursing through the party: opposition to Trump.

“No matter what your politics may be,” he told a crowd in Cedar Rapids, “I think we all understand that that is not the kind of person who should remain in the White House.”

Strategists suggested that the other front-runners — Biden, Buttigieg and Warren — were chasing Sanders. But a Des Moines Register/CNN/Mediacom poll that might have added some clarity with its planned Saturday night release was canceled after technical problems.

Uncertainty and fear undergirded the campaign’s final days. Some campaign aides worried that their candidate would be dealt a suffocating blow Monday night. And many voters and even some candidates said they were fearful about the party’s divisions at the very moment they hoped Democrats would be energized.

“We better not screw this up,” Sen. Amy Klobuchar (Minn.) said Saturday night.

Democratic anxiety has only been enhanced as impeachment proceedings clouded the approach of the caucuses. Trump, who continued to hammer Democratic candidates Sunday on Twitter and in a pre-Super Bowl interview on Fox, is expected to be acquitted by the Senate on Wednesday after debate that begins Monday, further emboldening him with a message from Senate Republicans that they will stand behind him even if he requests political assistance from foreign governments.

Impeachment has kept the senators who are running for president stuck in Washington for most of the past two weeks. Sanders, Warren and Klobuchar planned to return to the capital Sunday night, and hoped to make it back to Iowa Monday night to address supporters. Sen. Michael F. Bennet (Colo.), who has been campaigning in New Hampshire while most of his competitors were in Iowa, also planned to return to Washington late Sunday.

With a last day of campaigning that was unseasonably warm and sunny, candidates spread out to try to mobilize core supporters and convert as many undecided voters as they could.

Buttigieg was greeted by a crowd of more than 2,000 in Des Moines. Biden, who can sometimes struggle to fill a room, saw standing-room-only audiences, with 1,100 at one event. Warren’s staff had planned for roughly 350 people in Indianola, but more than three times that showed up — with space so tight that her top campaign advisers were temporarily barred by the fire marshal from entering.

Warren urged voters not to be afraid to vote for her and her agenda. After three years of the Trump presidency, she acknowledged, fear is prevalent among Democrats.

“The danger is real,” Warren said to the crowd. “Our democracy hangs in the balance. And at this moment, you will decide here in Iowa, what do we do? Do we pull back? Do we cower? Do we take the timid approach?” Warren said, drawing an implicit contrast between herself and more-moderate candidates in the race.

“Or do we fight back?” Warren continued. “Me? I am in this to fight back. That’s why I’m here. We fight back. Fighting back is an act of patriotism.”

Sanders skipped across the state, calling for sweeping change.

He framed Monday’s caucuses as the start of the “most consequential election, at least in the modern history of America.” He said his campaign was about beating Trump in November and “bringing fundamental reform to American society.”

Sanders stopped at campaign offices in three cities, giving abbreviated stump speeches to supporters preparing to canvass for him. It was a change of pace from the large rallies he has held in recent weeks, including a concert that drew an estimated 3,000 people Saturday night. He said the time had ended for simply expressing concerns about income inequality, health care and the environment.

“Now is the time to end the complaining. Now is the time for action. Action is tomorrow night,” Sanders said.

Sanders’s efforts to pressure Democrats to coalesce behind him have been made more complicated after some prominent supporters aggressively attacked some Democrats.

In Cedar Rapids on Saturday night, liberal filmmaker Michael Moore, who introduced Sanders at events across Iowa in recent days, took sharp aim at the Democratic National Committee, accusing its leaders of conspiring against Sanders and trying to boost former New York mayor Mike Bloomberg by changing qualifications for future debates.

“They did this because they are so nervous and worried about Bernie,” Moore alleged. A Sanders campaign spokesman did not immediately respond to a request for comment.

DNC spokeswoman Xochitl Hinojosa said Moore’s assertion was “totally false.” She added, “This is a conspiracy theory.”

Buttigieg began the day on a stage where he was introduced by Iowa City Mayor Bruce Teague, who had previously backed Sen. Cory Booker before the New Jersey Democrat dropped out of the race. He made a pointed reference to criticisms of Buttigieg’s difficulty attracting nonwhite voters.

“I don’t know if you’ve noticed,” Teague said. “But I’m black.”

Buttigieg has grown more animated and more emotive of late and his crowds, generally politely supportive, have become more raucous.

His campaign touted the 27 counties that voted for Barack Obama and Trump as evidence of his effort to reach more than just true-blue Democrats. He suggested that “a massive American majority” agrees on things like gun background checks or dealing with climate change, and suggested he is the candidate best equipped to activate it.

Two voters at the Coralville event told him they are Republicans voting for a Democrat for the first time.

“I’m not trying to trick anybody into thinking I’m more conservative than I am,” Buttigieg said Sunday, in a response to a different voter who asked how he plans to persuade Republicans. “I can’t say that we will agree 100 percent of the time, but I promise we will navigate those differences with respect and that everyone will be heard.”

When a voter told Buttigieg she was planning to vote for a Democrat for the first time and wondered why she should vote for him over Biden, Buttigieg said he was offering “just a different approach.”

“Every single time my party has won the White House in the last 50 years, we have done it with a candidate who is new in national politics, who is opening a door to a new generation of leadership, and who either doesn’t work in Washington or hasn’t been there very long,” he said. “That is how we win.”

Biden began the day courting voters in eastern Iowa, attempting to rally a crowd at a Catholic university by saying he was the candidate with not only the most widespread appeal but the one with the experience to unite a divided nation once in office.

“We have to unite not only the Democratic Party,” Biden told a standing-room-only crowd in Dubuque. “We have to unite the country. It’s the only way we’re going to be able to govern. We have to pull the country together, Democrats, independents, and Republicans. I refuse to accept the notion that we’re at war with the Republican Party.”

That bipartisan appeal, though, was tested later in the day when Sen. Joni Ernst (R-Iowa) raised the prospect that Republicans could attempt to impeach Biden because of the work his son did in Ukraine while Biden was vice president.

“I think this door of impeachable whatever has been opened,” Ernst said in an interview with Bloomberg News. “Joe Biden should be very careful what he’s asking for because, you know, we can have a situation where if it should ever be President Biden, that immediately, people, right the day after he would be elected would be saying, ‘Well, we’re going to impeach him.’ ”

Earlier in the day, some of Biden’s top advisers sought to play down any results in Iowa, arguing that the impact of a loss for him would be limited and suggesting voters consider the results of the first four states — not just the first.

“We’ve never said that we were going to run away with it. We’ve always said that this would be a fight, this would be a close race,” Symone Sanders, a senior adviser, said during a breakfast with reporters hosted by Bloomberg News. “We ain’t shocked.”

Biden advisers also attempted a difficult task: casting the former vice president, who has led in most national polls throughout the race, as the underdog.

“Since before vice president Biden got into this race — since before April 25 — people have been writing our campaign’s obituary,” Sanders said, adding that they remained confident. “Tuesday morning will be no different.”

Former senator Christopher Dodd, the Connecticut Democrat who has been campaigning with Biden, also sought to raise alarms about what a Sanders victory would mean to the party’s chances.

“Bernie describes himself as a socialist — certainly the president will have a field day with that argument,” Dodd said. “Bernie would have a very difficult time winning the election in November as our national candidate.”

He said Sanders would be unable to campaign with many Democrats and would “pose a serious threat” to the House majority.

“I hope no one is offended by that,” Dodd said. “That’s just the reality.”

Matt Viser contributed to this report.