CEDAR RAPIDS, Iowa — Some of Joe Biden’s leading rivals for the Democratic presidential nomination warned Sunday that embracing a cautious platform in 2020 could hand President Trump a second term, making an implicit argument against his candidacy at the largest gathering of White House hopefuls so far this year.

Speaking at the Iowa Democratic Party’s annual Hall of Fame celebration, Sen. Bernie Sanders (I-Vt.) asserted that a “middle ground strategy that antagonizes no one, that stands up to nobody and changes nothing,” amounts to a failed blueprint that could lead to Trump’s reelection.

Later, South Bend, Ind., Mayor Pete Buttigieg warned of dire electoral consequences. “We’re not going to win by playing it safe or promising to return to normal,” he said.

Neither Sanders nor Buttigieg mentioned Biden, who was noticeably missing from the festivities. He skipped them to attend his granddaughter’s high school graduation, according to his campaign advisers. Biden is headed to Iowa on Tuesday — the same day President Trump is scheduled to be in the state.

But their rhetoric built on arguments they advanced in speeches last week at the California Democratic Convention, which have been widely interpreted as a case against the former vice president. Biden has adopted more moderate views than many of his competitors, and he has argued that the Trump presidency is an aberration.

In casting doubts about his electability, his Democratic opponents are attempting to subvert what Biden’s champions believe is one of his chief strengths: They think he can defeat Trump by appealing to Democrats, moderates and some Republicans in a way no one else in the sprawling field can. Many Democratic voters have said that a candidate’s electability against Trump is the most important quality for them.

Nineteen candidates spoke at the celebration, delivering a rapid-fire series of speeches that were cut off by music when they extended beyond their allotted five minutes. Many emphasized the need to tackle climate change, ensure universal health care and protect abortion rights. But on those issues, they differed on the details.

The gathering came amid signs of a wide-open Democratic race in Iowa that appeared to be as unsettled as the broader battle for the Democratic nomination playing out across the country.

According to a Des Moines Register/CNN/Mediacom poll of Iowa released Saturday night, Biden led the Democratic pack with 24 percent. But Sanders (16 percent), Sen. Elizabeth Warren (D-Mass.) (15 percent) and Buttigieg (14 percent) were not far behind. Warren and Buttigieg had surged since the last survey, the poll said, and Sanders had declined.

With one exception, no one mentioned the leading candidate by name. “Joe Biden must really not like to travel,” quipped entrepreneur Andrew Yang, eliciting some groans in the crowd.

In more subtle ways, other candidates seemed to allude to Biden — or at least the wing of the party that he occupies.

“I’m not spending my time with high-dollar donors and with corporate lobbyists,” said Warren. “I’m spending my time with you. That’s how we build a grass-roots movement in America.” Biden has aggressively raised money from wealthy patrons, some of whom have given to Warren in the past.

“I don’t think there is room in our party for a Democratic candidate who does not support women’s full reproductive freedom,” said Sen. Kirsten Gillibrand (D-N.Y.). Biden came under heavy criticism from Democrats last week for opposing the repeal of a law restricting the use of federal funds for abortions. After the outcry, he swiftly reversed himself.

The pageantry of the event provided a vivid snapshot of how expansive and wide-open the Democratic field has become. And it offered a preview of the jockeying that is likely to unfold on the sidelines of the upcoming televised debates, which will begin later this month.

Hours before the speeches kicked off inside a massive hotel ballroom here, the campaigns packed the sidewalks outside with supporters who competed to be the loudest and most enthusiastic.

“It’s time for a woman in the White House!” chanted supporters of Sen. Kamala D. Harris (D-Calif.) as they bobbed campaign signs above their heads. Across the street stood supporters of former Texas congressman Beto O’Rourke, dressed in dark colors. On the next block, Sen. Cory Booker’s team spelled out his first name in giant letters.

A shirtless man marched up and down the sidewalk outside the event. He held a sign that read: “Yes, there are nineteen of them. No, they did not all come in the same car.”

Inside, the candidates sought to distinguish themselves from others in the expansive pack.

Buttigieg argued in his speech that Republicans do not have a monopoly on American values and touched on a topic that most other Democratic contenders have rarely broached: religion.

“God does not belong to any political party — least of all the one that produced this current president,” he said.

Some candidates acknowledged the massive field with humor or pop culture references.

“It’s been a pleasure speed dating with you tonight,” said Rep. Tim Ryan (D-Ohio). “If you want a second date, go to timryanforamerica.com.”

Rep. Eric Swalwell (D-Calif.) likened the huge crop of candidates to “The Avengers.” By comparison, the large Republican field in 2016 was “The Hunger Games,” he said.

At least one candidate aired a grievance. “Thanks for not changing the rules this week for who gets to be on this stage,” said Montana Gov. Steve Bullock, who is at serious risk of being shut out of the first debates due to the Democratic National Committee’s decision to narrow the qualification criteria it had initially announced.

Lesser-known candidates tried to leave a lasting impression on the crush of Democratic activists and members of the news media that descended on Cedar Rapids this weekend. Their efforts weren’t always smooth.

Young staffers for former Maryland congressman John Delaney’s campaign frantically tried to blow up a small blimp bearing the candidate’s logo to float above the crowd of Democrats rallying ahead of the main event. Can after can of helium was deposited, at least a half dozen in all, trying to get the craft off the ground, but it simply wouldn’t float.

Finally, a Delaney staffer urged them to carry the blimp above their heads. “Hoist it up,” he said. And off they marched across the street, long after many Democrats had already headed inside.

Holly Bailey contributed to this report.