CEDAR FALLS, Iowa >> As Bernie Sanders exudes confidence in his ability to win next week’s Iowa caucuses, his moderate rivals are struggling with how — and whether — to directly take on the progressive Vermont senator who some Democrats worry won’t be able to defeat President Donald Trump.

Former Vice President Joe Biden jabbed Sanders — without naming him — on the multitrillion-dollar cost of his most ambitious proposals, such as single-payer health insurance, tuition-free college and sweeping climate action.

“I don’t think you win votes doing that,” Biden said today in Cedar Falls. “I think it scares the living devil out of people.”

Across the state in Boone, Pete Buttigieg, the former mayor of South Bend, Indiana, also raised questions about Sanders’ electability — but only when pressed. Asked if Sanders was being evasive during a recent interview where he admitted he couldn’t put a price tag on his health care plan, Buttigieg said only that it was “striking that there’s no explanation of how this is supposed to work.”

“My focus is on the fact that my campaign is in the best position to beat Donald Trump,” Buttigieg told reporters when asked whether he believes Sanders would lose a general election. “My focus — while reporters, I’m sure, are eager to get me to do otherwise — is to remain as focused as possible on my own campaign.”

Sanders has long identified as a democratic socialist, and the prospect that he could win the caucuses and gain momentum heading into later contests has alarmed the establishment wing of the Democratic Party. But that anxiety was hard to detect on the campaign trail as Biden and Buttigieg, two of the leading moderate candidates, declined to take him head-on, opting instead to speak about the need to unify the party and the urgency of beating Trump.

That’s frustrating to those who would like to see the candidates take a stronger stand.

“It is shocking that no one besides us and a handful of others are willing to say what is evidently true, which is that he is a front-runner to win, and if he wins it’s going to be incredibly hard to beat Trump and hang onto our House majority,” said Matt Bennett, a co-founder of moderate think tank Third Way.

A Sanders nomination is far from a sure thing. A New York Times/Siena College poll released Saturday showed Sanders with a slight edge in Iowa, though polls also show that Buttigieg, Biden and Massachusetts Sen. Elizabeth Warren remain in the top tier there. In New Hampshire, several recent polls put Sanders out front, with Warren and the other top candidates lagging behind.

That position presents a delicate challenge for Biden and Buttigieg, who know that Sanders will be emboldened if he wins the Iowa caucuses next Monday and the New Hampshire primary the following week. But each must also consider how he can stitch together a Democratic coalition in November with himself as the nominee. That means avoiding the kind of open animosity that party leaders believe hurt Hillary Clinton against Trump in 2016, well after she dispatched Sanders in an extended primary fight.

While they largely avoided talking about Sanders during campaign events, the moderate candidates displayed less reluctance to knock Sanders in appeals to potential donors. Shortly after his comments to reporters today, Buttigieg’s campaign issued a fundraising plea, warning of Sanders’ strength and declaring, “we risk nominating a candidate who cannot beat Donald Trump in November.”

Biden’s campaign also sent a fundraising solicitation citing Sanders’ strength: “We’re on the cusp of taking the lead in New Hampshire, but Bernie Sanders has an enormous war chest, and his campaign is on the attack against Vice President Joe Biden.”

Biden’s campaign has doubled down in recent weeks on the argument that he is the best-positioned Democrat to defeat Trump and help the party in down-ballot races. But his aides and supporters insist their strategy isn’t explicitly about Sanders.

“I think people are thinking about electability more,” said Iowa Attorney General Tom Miller, a Biden supporter. But he stopped short of saying there’s a “fear” of Sanders as nominee. “When Iowans start thinking about the fall … people are starting to see that Joe would be much stronger than Bernie.”

Another candidate, Michael Bloomberg, was less subtle in striking a contrast with Sanders by taking his campaign to Sanders’ backyard. Appearing in Burlington, Vermont, the former New York City mayor was asked by a reporter why he’d be a better candidate than Sanders to take on Trump. He responded that he’d done it before, with his policy crusades for gun control and against vaping.

While Sanders is “the local favorite,” Bloomberg joked, he argued that his business experience made him the strongest candidate and dismissed the senator’s calls for a political revolution.

“In terms of a lot of people who think this country should evolve rather than have a revolution, I would be their choice,” Bloomberg said.

Attacking Sanders has proven difficult. He and Warren had a sort of non-aggression pact until she alleged recently that Sanders had told her privately in 2018 that a woman cannot win the presidency in 2020. Sanders disputes that account. Warren sticks by it, but raising the matter hasn’t noticeably propelled her in the race.

Still, Sanders’ campaign seemed to be readying for a counterstrike over the weekend, preemptively warning supporters that, as New York Rep. Alexandria Ocasio-Cortez said at a canvass launch in Ankeny, “things are gonna get crazy.”

“I don’t even know what’s gonna happen,” she said. “And that’s why we’ve gotta stay focused and committed as possible.”

Documentary filmmaker Michael Moore told supporters at a Saturday night rally that “the knives are out” for Sanders.

Ian Sams, who was spokesman for two previous Sanders opponents, Clinton and California Sen. Kamala Harris, said at this point he doesn’t expect candidates to go directly after Sanders. But if he wins Iowa, those dynamics could quickly shift.

“His main rivals will need to engage if they want to stop him because, in a primary, he’s unlikely to be a candidate who defeats himself,” he said.

Some Democratic voters think Biden or Buttigieg will have to soon make their case more explicitly.

“Eventually I think Biden is going to have to, because maybe in the end they may end up the last two standing,” said Kristi Marchesani, a 46-year-old Buttigieg supporter who came to see the former vice president today at the University of Northern Iowa.