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 and their shot at winning the Senate.

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.

“I’m the first Democrat to win in my district since 1958,” said Representative Dean Phillips of Minnesota, one of the front-liners who is backing his home state senator, Amy Klobuchar, in the presidential contest. “I attracted a lot of independent and moderate Republican support, many of whom probably voted for a Democrat for the first time in a long time. And while I respect Bernie Sanders as a senator, as a candidate, his candidacy is very challenging for people who come from districts like mine.”