WASHINGTON — As Bernie Sanders emerges as the leader in the race for the Democratic presidential nomination, his rise is generating fears among centrist Democrats that the apparent leftward shift of their party could cost them not only a chance to retake the White House, but also their hold on the majority in the House of Representatives.
The anxiety is particularly acute on Capitol Hill among a small but politically important group of freshman Democrats who helped their party win control of the House in 2018 by flipping Republican seats in districts that President Trump won in 2016. Now, they fear that having a self-declared democratic socialist at the top of the ticket could doom their re-election chances in November.
Members of the group of about three dozen — often called “front-liners” or “majority-makers”— have toiled to carve out political identities distinct from their party’s progressive base, and most are already facing competitive re-election challenges from Republicans who bill them as radicals who have empowered a far-left agenda in Congress.
Eight of them, including Representatives Haley Stevens of Michigan, Max Rose of New York and Lucy McBath of Georgia, have endorsed Michael R. Bloomberg, the former New York City mayor. Others, including several military veterans — Representatives Conor Lamb and Chrissy Houlahan of Pennsylvania, and Elaine Luria of Virginia — are coalescing around former Vice President Joseph R. Biden Jr.
Few are eager to publicly articulate their fears of a Sanders nomination, reluctant to call attention to the divisions within their party or risk alienating a potential nominee, but several of them privately described a sense of foreboding that has set in over the past two weeks as Sanders has emerged as the top finisher in the first two contests of the Democratic race.
“There is a growing concern among especially those of us on the front lines that we will not only lose the White House but the House of Representatives,” one of them said in an interview, insisting on anonymity to avoid criticizing a potential nominee.
Two former chairmen of the party’s House campaign arm — Steve Israel, who has endorsed Mr. Biden, and Rahm Emanuel, who is not backing any candidate — say the lawmakers are right to be concerned. Mr. Emanuel, the former mayor of Chicago, led Democrats to retake the House in 2006 using a playbook he called “metropolitan majority” — a “center-left” agenda aimed at uniting urban and suburban voters.
“Back in 2006, we created Red to Blue as a political entity,” Mr. Emanuel said, referring to a program Democrats made to help candidates flip Republican seats. “We never established or created ‘blue to deep blue.’ That’s not how you create majorities.”
He said governorships, the Senate and state legislatures — which govern redistricting and thus exert powerful influence over the political makeup of Congress — are also at stake.
“Every time we have won the White House, gained seats in the House and the Senate and the state capitals, we have run based on a model that has proved itself in presidential years, and off presidential years,” he said. “The question is: Do you want to take that playbook and throw it out?”
Mr. Israel sees two reasons for concern: The race for president will be won or lost in seven swing states and about 20 to 30 swing counties. And the “down ballot effect” — the tendency for the candidate at the top of the ticket to dominate voters’ assessments of other candidates of his or her party — is very strong in a presidential race.
“Donald Trump will paint every Democrat — whether they’re running for U.S. Senate or county sheriff — as a socialist, as a ‘Bernie Sanders socialist,’” he said, “and that’s a tough deal in a lot of these districts.”
Still, the race is young. While Mr. Sanders won in New Hampshire on Tuesday night, the majority of voters — nearly 53 percent — picked a trio of more moderate candidates: Mr. Biden; Pete Buttigieg, the former mayor of South Bend, Indiana; and Senator Amy Klobuchar of Minnesota. And three candidates, Senator Michael Bennet of Colorado; Deval Patrick, the former governor of Massachusetts; and the businessman Andrew Yang, just dropped out.
“This race is going to narrow,” Mr. Israel said. “The two lanes ultimately will be far-left and center-left, but right now the opening arguments seem to favor the pragmatic lane.”
Party leaders insist their front-liners will be fine no matter who is at the top of the ticket. Representative Cheri Bustos, who leads the House Democrats’ campaign arm — and represents a district in Illinois won by Mr. Trump — insisted the discussion is premature.
“We have a long way to go before we know who the nominee is,” she told reporters in the Capitol.
But for many of these lawmakers, the decision about whom to support is not one they can put off. The front-liners are coveted surrogates in their home districts because they have the backing of the independents that any Democrat would need to take on President Trump.
They also are fielding frequent appeals from the contenders for their endorsements. So with less than a month to go before Super Tuesday, they are feeling pressed to decide on a candidate — one with the best chance of toppling Mr. Sanders so that they do not have to run alongside him.
Many are also raising money furiously. Mr. Rose, for instance, has already raised more than $3 million, an extraordinary amount for a freshman. In interviews, he and Ms. Stevens steered clear of even mentioning Mr. Sanders’s name.
But in explaining why she endorsed Mr. Bloomberg, Ms. Stevens made it clear she did not believe that Mr. Sanders, the progressive from Vermont, had a profile that would appeal to her constituents.
“What I think is going to resonate in my district is somebody who is a world-class business leader or a government leader,” she said, “somebody who has lead a city that is bigger than the populations of certain states.”