Bernie Sanders has seized a commanding position in the Democratic presidential race, building a diverse coalition that is driving his liberal movement toward the cusp of a takeover of a major political party.

The senator’s ascendancy, though years in the making, is forcing a sudden reckoning in the Democratic Party’s hierarchy, as centrist politicians and their wealthy benefactors grapple with the upheaval brought by an electorate not only hungry to defeat President Trump, but also clamoring for radical change.

Subscribe to the Post Most newsletter: Today’s most popular stories on The Washington Post

Following Sanders’s resounding victory in Saturday’s Nevada caucuses, and with polls showing him on the rise, Democrats are entering a season of open warfare over whether Sanders (I-Vt.) is equipped to beat Trump in what could be a brutal general election. The senator and his allies insist he could, but his detractors say he is too polarizing to win in November — and could severely cost Democrats in congressional or state races if Republicans use Sanders’s self-description as a democratic socialist to paint all Democrats as extreme.

The Sanders insurgency is the culmination of grievances that have simmered for the past decade among liberals who say Washington has all but ignored the problems of income inequality, health-care access and climate change.

© Paul Ratje/AFP/Getty Images Sen. Bernie Sanders (I-Vt.), with his wife Jane, waves to the crowd Saturday at a rally in El Paso.

A headstrong, 78-year-old senator, Sanders has galvanized his supporters with an unwavering commitment to their shared cause and forceful critiques of the “billionaire class.” They in turn see him, despite his unorthodox persona, as a weapon against a governing class that has failed them.

On the campaign trail, there is an unusual intensity to Sanders’s performances, reminiscent of the energy that built around Trump on the right during his 2016 rise. Sanders has emerged as a movement candidate, with his rallies coast to coast drawing thousands of people who wait for hours to see him.

Sanders’s stump speech is a liberal wish list — passing a Green New Deal to combat climate change; wiping out student debt and paying for it by taxing Wall Street; raising the federal minimum wage to $15 per hour; reforming immigration laws to protect the undocumented; nominating liberals to the Supreme Court and protecting abortion rights; and, of course, his signature health-care idea, Medicare-for-all, which has become a rallying cry on the left.

“People who have been locked out of power are speaking up about corporate influence over the issues that matter in their lives,” said Abdul El-Sayed, a Sanders ally and liberal organizer who ran unsuccessfully for Michigan governor in 2018. “What you’re seeing is a necessary and natural readjustment in the Democratic Party.”

© Eric Gay/AP Sanders supporters cheer as they watch results of the Nevada caucuses Saturday at a rally in San Antonio.

Sanders’s emphatic win in Nevada illustrated his potential to expand his coalition far beyond the ceiling of 25 percent or 30 percent that many party-establishment figures and commentators assumed he had. In Nevada, Sanders won with 29 percent of whites, 51 percent of Hispanics and 27 percent of blacks, according to entrance polls of Democratic caucus-goers. He won a staggering 65 percent of caucus-goers under 30 years old, and he carried every other age group except for caucus-goers over 65 years old, which former vice president Joe Biden won.

“In Nevada, we have just put together a multigenerational, multiracial coalition which is going to not only win in Nevada, it’s going to sweep this country,” Sanders said at his rally in San Antonio on Saturday.

“We are bringing our people together — black and white and Latino, Native American, Asian American, gay and straight,” Sanders added. “We are bringing our people together around an agenda that works for the working people of this country.”

Sanders’s dominance among young people, his supporters say, signals his ability to energize this potentially important demographic in November.

“Disregard electability,” said Isabel Lozoya, 19, a Texas State University student who drove for an hour on Saturday to see Sanders campaign in San Antonio. “It should be about picking somebody you really believe in as opposed to somebody you think other people will believe in.”

The race for the nomination is just getting started and remains fluid, with a half-dozen contenders still running, although Sanders has clear momentum after winning Nevada and the New Hampshire primary, while finishing second in the Iowa caucuses by a tiny margin.

The next primary is on Saturday in South Carolina, where the latest polls show Biden leading and Sanders running close behind. The Super Tuesday contests on March 3 may be decisive, with voters in California, Texas and 12 other states determining approximately one-third of the nearly 4,000 pledged delegates to be awarded by primaries and caucuses.

© Mark Felix/AFP/Getty Images Sanders speaks during a rally in Houston on Sunday.

Some other candidates have stepped up their attacks on Sanders in urgent hopes of blunting his rise. Pete Buttigieg, the former mayor of South Bend, Ind., has been one of the most aggressive, warning in a speech Saturday night that Sanders as the party standard-bearer could be disastrous for other Democrats on the November ballot.

“Before we rush to nominate Senator Sanders in our one shot to take on this president, let us take a sober look at what is at stake for our party, for our values and for those with the most to lose,” said Buttigieg, who ran third in Nevada following a win in Iowa and a second-place finish in New Hampshire.

Sanders is bracing for a harsher assault to come from his Democratic rivals, including at Tuesday night’s CBS News debate in South Carolina.

“To finally be seeing it all start to catch on is powerful, but he knows they’re going to throw the kitchen sink at him. He’s a realist,” said Sanders’s friend Ben Cohen, the co-founder of Ben & Jerry’s ice cream and a Vermont-based liberal activist.

Some Democratic leaders are sounding the alarm about the party’s viability in the November election with Sanders atop the ticket. House Majority Whip James E. Clyburn (D-S.C.), an ally of House Speaker Nancy Pelosi (D-Calif.), said Sunday that Sanders could jeopardize the party’s House majority.

“I think it would be a real burden for us in these states or congressional districts that we have to do well in,” Clyburn said on ABC’s “This Week.” “If you look at how well we did the last time [in the 2018 midterm elections] and look at the congressional districts, these were not liberal or what you might call progressive districts. These were basically moderate and conservative districts that we did well in.”

© Callaghan O’hare/Reuters A girl holds a campaign sign as she waits for Sanders to arrive Saturday in San Antonio.

Still, the Rev. Jesse L. Jackson, the civil rights leader who unsuccessfully sought the Democratic presidential nomination in 1984 and 1988 with a message about “economic violence,” said it is clear to him that the party’s liberal wing is asserting control.

“They represent the direction of the party,” said Jackson, who said he has spoken recently with Sanders and Sen. Elizabeth Warren (D-Mass.). “They’re speaking to the pain that people feel. And Democrats are beginning to understand that democratic socialism doesn’t mean Eastern European socialism.”

Randi Weingarten, president of the American Federation of Teachers, compared the disruptive mood of Democratic voters this year to the right-wing tea party movement in 2010.

“They want to shake things up. There is a sense that things are broken, and prioritizing working families has to be at the center of the economic system,” said Weingarten, whose union has not endorsed a candidate but last week approved its members to support Sanders, Warren or Biden.

Former Wisconsin governor Jim Doyle said Sanders “definitely has tapped into the kind of youthful enthusiasm and idealism that’s been at the heart of the Democratic Party for a long, long time.”

“Democrats are always best when the race is one in which it’s change versus status quo, and the Democrats are change,” Doyle added. “You go back historically to Roosevelt, to Kennedy, to Carter, to Clinton, to Obama. That’s how Democrats win.”

Sanders is trying to counter the assumption of many in the so-called Democratic establishment that he is too liberal to win a general election.

“Some of the folks in the corporate media are getting a little bit nervous,” Sanders said at a rally Sunday afternoon in Houston before an enthusiastic crowd of more than 6,200 at the University of Houston. “And they say Bernie can’t beat Trump.”

Sanders then listed the results of a few recent polls that he says show him defeating the president head-to-head nationally as well as in such states as Michigan, Pennsylvania and Wisconsin.

© Callaghan O'hare/Reuters Sanders speaks at a campaign rally in Houston on Sunday.

Steve Rosenthal, a veteran Democratic labor strategist who has been focused on mobilizing working-class white voters in a trio of battleground states — Michigan, Pennsylvania and Wisconsin — said it would be foolish to discount Sanders’s appeal there.

“The establishment, which I guess I’m a part of after all these years, seems to know as much about electability as a donkey knows about calculus,” Rosenthal said. “We always get it wrong. . . . The voters are going to tell us who’s electable.”

Rep. Ro Khanna (D-Calif.), a national co-chair of the Sanders campaign, said it is expanding its outreach to a broader cross-section of voters.

“He wants to build a coalition like Bobby Kennedy or F.D.R. did — one that is racially diverse and reaches out to everyone,” Khanna said. “We’re going to make a very concerted effort over the next few months to bring all the wings of the Democratic Party onboard.”

Working to Sanders’s advantage is the persistence of several more moderate candidates — Biden, Buttigieg, Sen. Amy Klobuchar (D-Minn.) and former New York mayor Mike Bloomberg — who appear to be competing for many of the same voters and jockeying to survive as the lone alternative to Sanders.

For the proudly liberal and activist wing of the Democratic Party, Sanders’s ascent has been welcomed as a potentially historic development. Robert Reich, the liberal former labor secretary and a professor of public policy at the University of California at Berkeley, said this moment can be traced directly back to the 2008 financial crisis, which he called a galvanizing event that led to a surge in anti-establishment fervor.

“This isn’t like 1972,” when liberal Sen. George McGovern (S.D.) won the Democratic nomination and collapsed in the general election against Richard M. Nixon, Reich said. “In 1972, America’s middle class was still growing. What you see here is a middle class responding to not having a raise in 40 years.”

Even if he fails to secure the nomination outright at the Democratic National Convention in Milwaukee this summer, Sanders is unlikely to go away quietly, aides and friends said. They suggested that Sanders’s long run and defiant exit from the 2016 race — doggedly carrying on with his calls for a political revolution until the final primary, weeks after Hillary Clinton had effectively sewn up the nomination — was a revealing glimpse into his character and his desire to move the party to the left.

“He doesn’t quit,” said Sanders confidant and political adviser Jeff Weaver. “He’s campaigning to win.”

robert.costa@washpost.com

philip.rucker@washpost.com

Jenna Johnson and Sean Sullivan in Houston contributed to this report.