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…

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.