Democratic presidential candidates promised voters in Iowa on Saturday they would unify the party to take on President Donald Trump even as they kept up their criticism of each other and navigated the lingering divides from the 2016 campaign.

"I’m confident Americans, Republican voters, Democratic voters and independent voters want us to come together," former Vice President Joe Biden said in North Liberty. "I’m going to do whatever it takes to make progress in the areas that matter most."

About 20 miles away in Cedar Rapids, Massachusetts Sen. Elizabeth Warren updated her stump speech to include a more explicit call for unity.

"We’re down to the final strokes here," she said. "But we understand that, we will and we must come together as a party to beat Donald Trump and I’ve got a plan for that."

And Bernie Sanders insisted he would back the ultimate Democratic nominee even if it's not him.

"Let me say this so there’s no misunderstanding," the Vermont senator said in Indianola. "If we do not win, we will support the winner and I know that every other candidate will do the same."

On the eve of Monday's Iowa caucuses, the unity pledges marked an early — and urgent — effort to avoid the divides that some Democrats say helped Mr. Trump win the presidency in 2016. After a year of campaigning, most polls show a tight race between Mr. Biden, Ms. Warren, Mr. Sanders, and Pete Buttigieg, the former mayor of South Bend, Indiana. Those candidates, along with Sen. Amy Klobuchar of Minnesota and businessman Andrew Yang, crisscrossed Iowa on Saturday seeking a breakout victory that would propel them deeper into the contests that will decide the Democratic nomination.

Most surveys show the top four candidates jumbled at the top. And on Saturday, the final Des Moines Register poll — traditionally seen as the gold standard survey of the caucus electorate — was pulled from publication after questions about its methodology. The newspaper said Mr. Buttigieg may have been left off the list presented to a caucus-goer in at least one call.

To break out of the pack, the candidates have focused on a wide variety of policy issues ranging from free college tuition to the role of government in health care, criminal justice reform, gun control and solutions to climate change. But the biggest issue on the minds of many voters is landing on a candidate who can beat Trump.

Anxiety over the party's ability to unify grew over the past two weeks after Hillary Clinton, Mr. Sanders' 2016 primary rival, twice criticized the senator for not doing enough to bring Democrats together after their bruising battle.

The divide was on display Friday when Rep. Rashida Tlaib, D-Mich., appeared at a Sanders event in Iowa and booed at the mention of Ms. Clinton. She later issued a statement saying she "allowed my disappointment with Secretary Clinton's latest comments about Senator Sanders and his supporters get the best of me" and would "strive to come from a place of love and not react in the same way of those who are against what we are building in this country."

Faiz Shakir, Mr. Sanders' campaign manager, retweeted her statement and said: "We love your passion and conviction. Don't change."

Clinton spokesperson Nick Merrill called that comment "unbelievable."

"It’s not enough that she booed her party’s last nominee, but they’re making sure it’s clear she doesn’t have to regret it," Mr. Merrill tweeted.

The incident underscored questions about how Democrats can thread together clashing factions to develop a coalition that spans generations, races, and economic status to defeat Mr. Trump. With Mr. Sanders showing some signs of strength in Iowa, some voters said they wouldn't support him because of concerns he would divide the party.

Lisa Stolba, a retired school counselor from Cedar Rapids who attended a Biden event in North Liberty, said she’s undecided, considering Ms. Klobuchar, Mr. Buttigieg, Mr. Biden and Ms. Warren.

Mr. Sanders was not on the list for a few reasons. He was too "socialistic," she worried about his health and age — Mr. Biden’s too, she added — and she’s wasn't confident Mr. Sanders could unite the party. Ms. Stolba noted she was still smarting over whether he told Ms. Warren that a woman can’t win. Mr. Sanders denied the comment. "I think that he said it," she said.

But Ms. Stolba said she’ll still vote for Mr. Sanders if were the the nominee. "If he’s the only one, yeah, I’m not going to vote for Trump."

Mr. Sanders, for his part, has drawn crowds stretching into the thousands at his events, a clear demonstration of the enthusiasm surrounding his campaign. It's that energy, he and his supporters argue, that will bring the party together and inspire voters across the spectrum to support his candidacy in November.

Some candidates saw an opportunity in the infighting. Mr. Buttigieg has made generational change a central argument of his campaign and said Saturday that the divides underscore his point.

"I didn’t much enjoy as a Democrat living through the experience of 2016 and I want to make sure 2020 resembles 2016 as little as possible," he told reporters after a rally in Waterloo.

Mr. Buttigieg went on to emphasize that the candidates "are much more aligned than you would think."

But on stage later in the day, he revived his criticism of Mr. Biden and Mr. Sanders, outlining what he called "a respectful difference of approach among people who share the same values, share the same goals."

"The vice president is suggesting this is no time to take a risk on someone new," he said. "I’m suggesting this is no time to take a risk on trying to meet a fundamentally new challenge with a familiar playbook. It’s going to take something new."

Mr. Buttigieg criticized Mr. Sanders for "offering an approach that suggests it’s either revolution or it’s the status quo, and there’s nothing in between."

And although he made a pitch to unity, Mr. Biden still went after Mr. Sanders' policy checklist on the stump — though he avoided naming the Vermont senator.

"Talk is cheap. In politics, sometimes talk is very expensive, especially when you don’t tell people how you pay for what you say you’re going to do," Mr. Biden said in Waterloo. "Be straight. Tell the people what it’s going to cost."

Mr. Biden referenced some estimates that put Mr. Sanders' big-ticket proposals on single-payer health care, college debt forgiveness, and tuition-free college tuition at more than $60 trillion over 10 years. "I wonder why they don’t know how much it’s gonna cost," he added, an allusion to Mr. Sanders recently telling CBS News that he doesn't know exactly what his Medicare for All plan would cost the federal government.

Some voters said they felt that, regardless of the divisions within the party, Mr. Trump was enough of a unifying factor on his own that Democrats will have no issues after the primary. Tom Taiber from Waverly said he wasn't worried about Democrats coming together to rally around the party’s eventual nominee, even if the primary becomes divisive.

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"The family of Democrats, we’re going to have differences of opinion," he said.

This story was reported by The Associated Press. AP writers Julie Pace and Tom Beaumont in Waterloo, Iowa, and Bill Barrow in North Liberty, Iowa, contributed to this report.