For Rachel Conner, the 2018 election season has been a moment of revelation.
A 27-year-old social worker, Ms. Conner voted for Hillary Clinton in the 2016 primaries, spurning the more liberal Bernie Sanders, whom many of her peers backed. But Ms. Conner changed course in this year’s campaign for governor, after concluding that Democrats could only win with more daring messages on issues like public health and immigration.
And so on a recent Wednesday, she enlisted two other young women to volunteer for Abdul El-Sayed, a 33-year-old advocate of single-payer health care running an uphill race in Michigan to become the country’s first Muslim governor.
“They need to wake up and pay attention to what people actually want,” Ms. Conner said of Democratic leaders. “There are so many progressive policies that have widespread support that mainstream Democrats are not picking up on, or putting that stuff down and saying, ‘That wouldn’t really work.’”
Voters like Ms. Conner may not represent a controlling faction in the Democratic Party, at least not yet. But they are increasingly rattling primary elections around the country, and they promise to grow as a disruptive force in national elections as younger voters reject the traditional boundary lines of Democratic politics.
Energized to take on President Trump, these voters are also seeking to remake their own party as a ferocious — and ferociously liberal — opposition force. And many appear as focused on forcing progressive policies into the midterm debate as they are on defeating Republicans.
The impact of these activists in the 2018 election has been limited but revealing: Only about a sixth of Democratic congressional nominees so far have a formal affiliation with one of several important insurgent groups. Fifty-three of the 305 candidates have been endorsed by the Justice Democrats, the Working Families Party, the Progressive Change Campaign and Our Revolution, organizations that have helped propel challenges to Democratic incumbents.
House primary winners who were endorsed by at least one major the voters who make up the ascending coalition on the left have had an outsize effect on the national political conversation, driving the Democrats’ internal policy debates and putting pressure on party leaders unseen in previous campaigns.
Mark Brewer, a former longtime chairman of the Michigan Democratic Party, said “progressive energy” was rippling across the state. But Mr. Brewer, who backs Gretchen Whitmer, a former State Senate leader and the Democratic front-runner for governor, said Michigan Democrats were an ideologically diverse bunch and the party could not expect to win simply by running far to the left.
“There are a lot of moderate and even conservative Democrats in Michigan,” Mr. Brewer cautioned. “It’s always been a challenge for Democrats to hold that coalition together in the general election.”
Progressive activists have already upended one major election in Michigan, derailing a former federal prosecutor, Pat Miles, who was running for attorney general with the support of organized labor, by endorsing another lawyer, Dana Nessel, who litigated against Michigan’s gay marriage ban, at a party convention.
In more solidly Democratic parts of the country, younger progressives have battered entrenched political leaders, ousting veteran state legislators in Pennsylvania and Maryland and rejecting, in upstate New York, a congressional candidate recruited by the national party.
In Maryland, Democrats passed over several respected local officials to select Ben Jealous, a former N.A.A.C.P. president and an ally of Mr. Sanders who backs single-payer health care, as their nominee for governor. And in a climactic upset in New York last month, Alexandria Ocasio-Cortez, a 28-year-old Democratic socialist, felled Representative Joseph Crowley, the fourth-ranking Democrat in the House.
With about two months left in primary season, a handful of races remain where restive liberals could flout the Democratic establishment, demolishing archaic party machinery or pressuring Democrats in moderate areas to tack left. Beyond Mr. El-Sayed, there are also insurgents contesting primaries for governor in Florida and New York, for Senate in Delaware and for a smattering of House seats in states including Kansas, Massachusetts and Missouri.
The pressure from a new generation of confrontational progressives has put Democrats at the precipice of a sweeping transition, away from not only the centrist ethos of the Bill Clinton years but also, perhaps, from the consensus-oriented liberalism of Barack Obama. Less than a decade ago, Mr. Obama’s spokesman, Robert Gibbs, derided the “professional left” for making what he suggested were preposterous demands — like pressing for “Canadian health care.”
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.
Yet among Democratic stalwarts, there is a sometimes-rueful recognition that a cultural gulf separates them from the party’s next generation, much of which inhabits a world of freewheeling social media and countercultural podcasts that are wholly unfamiliar to older Democrats.
Evan Nowlin, a writer and barista supporting Mr. El-Sayed, said he had been motivated to volunteer by a podcast hosted by The Intercept, a left-leaning news site that has intensively covered challenges to the Democratic establishment.
The pressure from a new generation of confrontational progressives has put Democrats at the precipice of a sweeping transition away from the centrist and consensus-oriented liberalism of past.
Mr. Nowlin, a soft-spoken 26-year-old who supported Mr. Sanders in 2016, said the traditional Democratic leadership had plainly failed to inspire the country. “I think they’re generally spineless,” he said.
In some instances, the party’s rebels may be too brazen even for some of the candidates they have supported. The gradations of Democratic revolution were on display at an event in Brooklyn Tuesday celebrating the Working Families Party: Cynthia Nixon, the actor running in a September primary against Gov. Andrew M. Cuomo of New York, a more moderate Democrat, drew cheers hailing Ms. Ocasio-Cortez and her fellow Democratic socialists.
But Mr. Jealous, the Maryland nominee for governor, who is supported by Working Families and addressed the event, was warier of the socialist label. After embracing Ms. Nixon on stage but not quite endorsing her, Mr. Jealous chuckled at a question about the resurrection of Democratic socialism as a political identity.
“I’m a venture capitalist,” he said, noting his work as an investor. “I’m kind of like the last person to ask.”
Democrats in Maryland passed over several respected local officials to select another admired leader, Ben Jealous, a former N.A.A.C.P. president who backs single-payer health care.
In Michigan, however, Mr. El-Sayed is counting on a mood of ideological ambition to decide his primary: He remains an underdog, facing a well-funded rival in Ms. Whitmer, who is backed by powerful labor unions like the United Auto Workers. She has led in recent polls, while a third candidate, Shri Thanedar, a wealthy wild card, has complicated the race.
Aiming to build momentum, Mr. El-Sayed will campaign later this month with Ms. Ocasio-Cortez, to whom he linked himself in generation and political outlook. Ms. Ocasio-Cortez also campaigned in Kansas Friday for liberal House candidates and was slated for an event over the weekend for a primary challenger to a Democratic incumbent in Missouri, William Lacy Clay.
“The rise of somebody like Alexandria seems kind of obvious to somebody in our generation,” Mr. El-Sayed said in an interview, casting the moment in grand terms: “The machine, whether it is on the right or on the left, has assented to this broken system of corporate politics, and I think people are real frustrated about that.”
That mind-set unnerves Democratic veterans like Mr. Brewer, the former party chairman, in a state where they have long struggled to overcome a Republican machine aligned with the business community. Mr. Trump’s slim victory there exposed divisions between the national Democratic Party and many of the white union members on whose votes Michigan Democrats rely, underscoring Democrats’ tenuous position in 2018.
But within deep-blue precincts where Democratic insurgency appears strongest, talk of accommodating the center is in short supply.
In Massachusetts, where several incumbent House Democrats are facing feisty challenges, Michelle Wu, a 33-year-old member of the Boston City Council, said voters are demanding leaders who share their intense alarm about economic and racial inequality. Defying the local machine, she recently endorsed Ayanna Pressley, a fellow council member, in a primary against Representative Michael Capuano, a long-serving liberal.
“People want to believe we can take our own future into our hands,” Ms. Wu said.