This piece was originally published by The New York Times on August 25, 2020.

President Trump has flirted with the convoluted QAnon conspiracy theory for months. Last week, he gave a full embrace to its followers, telling reporters that its believers are patriots “who love our country.” Over 70 QAnon supporters have run for Congress as Republicans this year.

At least one of them, Marjorie Taylor Greene of Georgia, will probably join the House next year. Despite her QAnon advocacy and a history of racist and Islamophobic rants on social media, Mr. Trump hailed her as a “future Republican star.”

Most people refer to QAnon as a fringe movement. But that no longer makes sense: Under Mr. Trump, it has become part of the Republican mainstream — and that has troubling implications for the party’s future.

The QAnon movement routinely deploys racist and anti-Semitic tropes; it has even been identified as a potential domestic terrorism threat by the F.B.I. Yet a majority of Republican leaders have refrained from criticizing the president for legitimizing it. They do not seem bothered that the conspiracists, without a shred of proof, declare Democrats to be part of a “deep state” cabal of satanic, child-molesting cannibals and call for the president to imprison and execute them.

A handful of the least Trump-dependent Republicans have pushed back. Representative Liz Cheney of Wyoming called QAnon “dangerous lunacy that should have no place in American politics.” Representative Adam Kinzinger of Illinois termed it “a fabrication.” (For that, a Trump campaign official actually attacked him.)

Past Republican leaders knew better than to allow the pathogens of extreme conspiracy theories to infect the political bloodstream. The conservative movement that now dominates the party always had a dark side, but its leaders understood that conspiracy cults are lethal to the social trust on which democracy depends. They also realized it was in the best interest of their movement to marginalize its cranks and kooks.

QAnon presents the same kind of threat to the Republican Party that the far-right John Birch Society did in the late 1950s and ’60s. The Birchers trafficked in similar concepts of an evil elite corrupting and betraying the country. The society’s founder, the retired candy-maker Robert Welch, considered even President Dwight Eisenhower, a Republican, to be “a conscious, dedicated” Communist agent.

Birchers were never more than a small fraction of Republicans. But in some key states like California, they made up a sizable share of the party’s primary voters, donors and activists — what we now would call “the base.” Barry Goldwater became the 1964 Republican presidential nominee in part thanks to their efforts.

Even so, many Republican officials were willing to condemn the Birch Society, particularly after public revulsion over right-wing extremism (among other factors) contributed to the party’s catastrophic losses in the 1964 elections. Robert Taft Jr., the son and grandson of famous conservative politicians, emphasized that extremism was alien to Republican philosophy and that the party should not be “a home for the John Birch Society, the Ku Klux Klan, or any similar group.”

Ray Bliss, who became chairman of the Republican National Committee after Goldwater’s defeat, called upon Republicans to reject membership in any organization that “attempts to use the Republican Party for its own ends.” He singled out “irresponsible radicals such as Robert Welch.” Mr. Bliss repressed primary challenges from the right, worked to exclude Birchers from positions of power within the party and cooperated with moderate and conservative activists to prevent Phyllis Schlafly from winning the presidency of the National Federation of Republican Women.

William F. Buckley Jr., the pre-eminent conservative leader of the 1960s, tried to read the Birch Society out of his movement. He felt that the Birchers’ conspiracies discredited conservatism by making it seem “ridiculous and pathological.” The absurd claims also turned off a young generation who laughed along with Bob Dylan’s derisive “Talkin’ John Birch Paranoid Blues.”

Mr. Buckley insisted to a conservative critic that to govern and expand, the movement had to hold on to “moderate, wishy-washy conservatives” who made up a majority of Republicans. “If they think they are being asked to join a movement whose leadership believes the drivel of Robert Welch,” he warned, “they will pass by Crackpot Alley, and will not pause until they feel the embrace of those way over on the other side, the Liberals.”

Some historians consider Mr. Buckley’s efforts to purge the Birch Society to have been too little, too late, and the Republican Party undeniably played on social division and white backlash as it moved to the political right from the 1970s onward. But extremist groups like the Birchers were mostly relegated to the fringes for many years. That was the foundation for Republican presidential victories under Richard Nixon, Ronald Reagan and the two Bushes.

Today there are no gatekeepers of similar stature on the political right, partly because of structural factors that have undercut the power of parties. These include the decline of establishment-dominated conventions and the rise of primaries, the growth of outside spending groups and the proliferation of conservative media programming from the likes of Fox News and Sinclair. It’s also because of the unwillingness of Republican and conservative leaders, over at least the past two decades, to call out and challenge the growing extremism in their base.

There have been isolated exceptions. The party publicly condemned the former Klan wizard David Duke when he ran as a Republican in Louisiana. John McCain, as the Republican presidential nominee in 2008, rejected the birther conspiracy theorists (including Mr. Trump). Last year, after Steve King of Iowa defended white supremacy, the House Republican Caucus stripped him of his committee memberships.

But so long as Mr. Trump remains president, there will be no such actions against QAnon conspirators, no matter how extreme. Mr. Trump has done nothing to broaden the Republican Party’s appeal. His re-election strategy rests entirely on stoking his followers’ resentments — and Q believers who consider Democrats to be evil incarnate are integral to his hopes for success.

Senator Ben Sasse of Nebraska, another Republican who has displayed sporadic independence from Mr. Trump, called QAnon “nuts” and warned that if the Democrats retake control of the Senate, “garbage like this will be a big part of why they won.” Republicans lost their House majority in 2018 in part because college-educated suburbanites, who once reliably voted Republican, rejected Mr. Trump’s elevation of anger and division over competent government. If QAnon comes to define the Republican brand in the public mind, the party may never regain its lost swing voters.

If Mr. Trump loses in November and takes Republican control of the Senate down with him, the party’s leaders may rethink the path that led them to defeat. Perhaps they will remember the broad popularity the party enjoyed for decades after it resisted earlier versions of QAnon extremism and the ways in which that legacy has been squandered.

Photo Credit: Gage Skidmore via CC BY-SA 2.0.