That attitude now appears obsolete, on matters well beyond health policy. Corey Johnson, the progressive speaker of the New York City Council, who supported Mr. Crowley over Ms. Ocasio-Cortez, urged Democrats to recognize the intensity of “anger, fear and disappointment from people in our own party,” especially those new to the political process.

“They’re young, and a lot of them are folks that weren’t around or weren’t engaged when Obama ran for the first time,” Mr. Johnson, 36, said. “So this is their moment of: Let’s take our country back.”

In a source of relief to Democratic officials, the millennial-infused left has left a lighter mark in moderate areas where Republicans are defending their congressional majorities, and where bluntly left-wing candidates could struggle to win. In House races, Democrats have mainly picked nominees well to the left of center, but to the right of Mr. Sanders and Ms. Ocasio-Cortez.

Across most of the approximately 60 Republican-held districts that Democrats are contesting, primary voters have chosen candidates who seem to embody change — many of them women and minorities — but who have not necessarily endorsed positions like single-payer health care and abolishing the Immigration and Customs Enforcement Agency.

Some national Democrats remain skeptical that voters are focused on specific policy demands of the kind Mr. El-Sayed and Ms. Ocasio-Cortez have championed. Former Gov. Martin O’Malley of Maryland, a left-of-center Democrat who ran for president in 2016, suggested the party wants “new leaders and fresh ideas” more than hard-left ideology.

“Sometimes that may be filled by a leader who calls herself a Democratic socialist, and sometimes it’s not,” said Mr. O’Malley, reflecting on the political convulsion that touched his home state. “Sometimes it’s with a young person. Sometimes it’s with a retiree. Sometimes it’s with a vet.”

Several crucial Democratic victories since 2016 have also come with avowedly moderate standard-bearers, such as Senator Doug Jones of Alabama and Representative Conor Lamb of Pennsylvania, who won grueling special elections. And unlike hard-liners on the right, Democratic activists have not contested Senate primaries in conservative-leaning states where the majority is at stake, allowing centrists to run unimpeded in Arizona and Tennessee.