Many of the Democrats most at risk of losing their seats in Congress this year are helping Joe Biden make the most crucial argument for his campaign: that he is the best chance Democrats have of beating Donald Trump and winning control of Congress, and that one alternative — namely, Bernie Sanders — could bring disaster.

No one boasts more endorsements than Biden does from lawmakers included in the Democratic Congressional Campaign Committee’s Frontline program, which marshals resources to reelection efforts seen as vital to preserving the party’s House majority.

New endorsements came in rapid succession this month, led off by Rep. Abby Finkenauer, whose Iowa district is a battleground in the nation’s first caucus state. Next came Rep. Conor Lamb, who won a special election two years ago in a Pennsylvania region partial to Trump, and Virginia Rep. Elaine Luria. Rep. Colin Allred of Texas and New Jersey Rep. Tom Malinowski got on board last week after their initial preferences — former federal housing secretary Julián Castro and New Jersey Sen. Cory Booker, respectively — ended their White House bids. They joined Rep. Matt Cartwright of Pennsylvania, an early Biden backer, and Rep. Ami Bera, who co-chairs the Frontline program from a safer California district and endorsed Biden last month.

The rush of support stems from Biden’s aggressive campaigning for swing district Democrats in 2018 and his subsequently aggressive courtship of the winners he knew could be helpful to his 2020 run for president. And in some cases it reflects a growing concern among moderates that Sanders, the democratic socialist senator from Vermont, remains a frontrunner.

Biden made that argument in a roundabout way during an interview posted Sunday by the State, a South Carolina newspaper.

“I’m just asking the rhetorical question," Biden told the newspaper. "Bernie’s at the top of the ticket in North and South Carolina, or [Sen. Elizabeth] Warren’s at the top of the ticket. How many Democrats down the line do you think are going to win? It’s just practical.”

Bera, in a telephone interview with BuzzFeed News, was more explicit: “I think that if Bernie Sanders is our nominee, it’ll make a lot of these Trump districts that we picked up extremely competitive and probably does put our House majority in jeopardy. If you nominate anyone else, Bernie Sanders or even Sen. Warren, it’s going to be hard for me. … I’ve heard that multiple times, and I think other members in these competitive districts are hearing that same thing.”

A Sanders spokesperson did not respond to a request for comment.

Bera’s remarks came days after Jim Messina — the manager of Barack Obama and Biden’s 2012 reelection campaign who recently attended a Biden fundraiser but has not endorsed a 2020 candidate — raised concerns about Sanders in an interview with Politico.

“If I were a campaign manager for Donald Trump and I look at the field, I would very much want to run against Bernie Sanders,” Messina said. “I think the contrast is the best. He can say, ‘I’m a business guy, the economy’s good and this guy’s a socialist.’ I think that contrast for Trump is likely one that he’d be excited about in a way that he wouldn’t be as excited about Biden or potentially Mayor Pete [Buttigieg] or some of the more Midwestern moderate candidates.”

Bera agrees with Messina. He said he has heard from independent and Republican-leaning voters in his district — where Hillary Clinton beat Trump convincingly in 2016 — who favor Biden and would not support Sanders. “I don’t think I’ve ever heard any of these independent voters say, ‘Hey, if you nominate Bernie Sanders, I’m going to vote for him.’”

A national Quinnipiac University poll from December that found Biden leading the primary field by a comfortable margin also found a plurality of Democrats and Democratic-leaning voters (44%) believed he stood the best chance of beating Trump. Sanders, at 15%, was a distant second. But early data hasn’t always cleanly validated Biden’s electability case.

General election polls in electoral battlegrounds are all over the place. A recent Florida Atlantic University survey measured Sanders with a bigger lead over Trump than Biden in that state. A new EPIC-MRA poll in Michigan found Biden up on Trump by 6 points, Sanders up by 5. (Former New York mayor Michael Bloomberg was up by 7.) Biden, for now, appears more competitive in Pennsylvania, Wisconsin, and Texas. Allred and Lamb both see Biden as the Democrat most likely to win their states this fall, though Texas is a reach for whoever the nominee is.

“There’s been so much public polling out there that shows Biden not only does better against Trump nationally, but when you get into the real swing states of Wisconsin, Michigan, not only does he do better, there are some instances where the margin is quite large between him and other candidates,” Biden pollster John Anzalone told BuzzFeed News.

Anzalone declined to share internal poll numbers the campaign might be drawing on to attract Frontline members and others in tough House districts. (Newly announced Biden backer Sean Patrick Maloney, for example, comes from a New York district that narrowly favored Trump.) Regardless of data, he said, these lawmakers are more ideologically aligned with Biden.

“These are really smart people,” Anzalone said. “I don’t think they’re just putting their finger to the wind. Most of them are moderates.”

“The real story of 2018 wasn’t the AOCs,” Anzalone added, referring to Sanders ally Alexadria Ocasio-Cortez, who unseated a moderate Democratic incumbent in New York. “Most of the people who helped take back the House were moderates, and Biden is a moderate.”

Lamb, in a telephone interview, cast his differences with Sanders as a matter of substance, noting the senator’s push for eliminating private insurance through Medicare for All and his support for a total ban on fracking — a promising industry in his Pittsburgh-area district.

“While I think a lot of his ideas have a good heart behind them,” Lamb said of Sanders, “I don’t think I’d be doing my job as a representative to these people if I supported someone who wanted to destroy their livelihood.”