With the fate of the progressive agenda depending on the support of Senator Joe Manchin III, who said again on Sunday that he would not abandon the filibuster to pass an expansive voting rights bill, interest groups and activists are gearing up for a full push to try to sway the moderate Democrat. It would be enough to make almost any Democratic politician in the country squirm.But probably not a Democrat from West Virginia.None of the demographic groups that animate today’s Democratic coalition are well-represented in the state. Black, Hispanic, college-educated, young, urban and professional voters all represent a much smaller share of the electorate in West Virginia than just about anywhere else.White voters without a four-year degree, Donald Trump’s demographic base, made up 69 percent of voters there in 2020, according to census data, the highest in the country. Mr. Trump won West Virginia with 69 percent of the vote in 2020, more than in every state but Wyoming.With those sorts of numbers, it’s hard to understand how Mr. Manchin is a Democratic senator at all in today’s polarized era. His state voted for Mr. Trump by 39 points last November; no other member of the House or Senate represents a state carried by the other party’s presidential candidate by more than 16 points.Yet Mr. Manchin’s unique ability to survive in West Virginia is the last vestige of the state’s once-reliable New Deal Democratic tradition, dating to old industrial-era fights over workers’ wages, rights and safety. It was one of the most reliably Democratic states of the second half of the 20th century, voting in defeat for Adlai Stevenson in 1952, Hubert Humphrey, Jimmy Carter in 1980 and Michael Dukakis. The so-called Republican “Southern strategy” yielded no inroads there.But Democrats began to lose their grip on the state during the 1990s, at least at the presidential level. In a way, West Virginia voters have been thwarting progressive hopes ever since. The promise of a new progressive, governing majority always rested on the assumption that the Democrats would retain enough support among white, working-class voters, especially in the places where New Deal labor liberalism ran the strongest. They did not.By the late 1990s, the old New Deal labor Democrats no longer defined the party nationally. And when in conflict, the party’s growing left-liberal wing prevailed over working-class interests: New environmental regulations hurt West Virginia’s already faltering coal industry; new gun control laws put Democrat at odds with an electorate where most voting households own a gun (in the 2018 exit polls, 78 percent of voters said someone in their household owned a gun).In 2000, George W. Bush won the state. If Al Gore had captured its six electoral votes instead, he would have been the president. Most analysts, however, saw the Bush win as an anomaly. In “The Emerging Democratic Majority,” a book arguing that Democrats were on the cusp of an enduring advantage despite their defeat in the 2000 election, the authors projected West Virginia as a “Lean Democratic” state.In retrospect, the loss of West Virginia was no anomaly. Democrats lost ground in every presidential election from 1996 until 2016, by which point the state had shifted nearly a net 60 points toward the Republicans over 20 years. It’s part of a broader pattern, not only in the United States but also across the world: The old bastions of the industrial-era left have chosen the populist right over the new progressive left.Nearly two decades later, Mr. Manchin is the only Democrat who holds statewide office in West Virginia. He might not have won the seat at all if he wasn’t a popular governor when he ran for the Senate in 2010. To win, he ran an advertisement promising to take “dead-aim” at the Obama-era “cap and trade” bill, which hobbled the party throughout coal country. The ad showed him shooting a copy of the legislation, which aimed to set limits on greenhouse gas emissions but created a market for companies that cut pollution quickly to sell allowances to high polluters.In 2018, Mr. Manchin may have only won re-election because of the favorable national environment that helped Democrats retake the House.Today Republicans have the registration advantage in West Virginia for the first time since 1932, when Franklin D. Roosevelt won the presidency. Democrats had a 14-point voter registration advantage in West Virginia in 2016, when Mr. Trump won by 42 points — the best showing of any presidential candidate from either party in the history of the state. They still had a 9-point advantage in 2018, when Mr. Manchin won re-election by 3 percentage points.It is far too soon to evaluate Mr. Manchin’s chances in 2024, but early indications are not promising.Mr. Manchin voted to convict Mr. Trump at his impeachment trial in February, and he has been front and center in major legislative debates over enacting President Biden’s agenda.According to the Cooperative Election Study, a prominent academic survey, Mr. Manchin had just a 33 percent approval rating in October 2020, while 51 percent disapprove of his performance.Mr. Manchin’s departure, whether in 2024 or thereafter, will mark the end of an era. There will be no Senate Democrat whose electoral history and coalition are so completely at odds with the new activist base of the party. Progressives will be free from the burden of trying to lure a senator with such a conservative voting base.But Democrats will also be weaker, at least in their numbers in the Senate, for not having found a way to forge a durable alliance with some of the most reliable Democratic voters of the 20th century.