Republican leaders were shocked, shocked to learn about revelations that Rep. Marjorie Taylor Greene (R-Ga.) once approved of calls for the execution of Democrats. They are so troubled by this that they plan to sit her down and give her a slap on the wrist with a little plastic ruler.

In so doing, they will be reminding us of a story about the GOP and conservative movement that goes back at least a half century: Their failure to adequately police the extremists in their midst.

House Minority Leader Kevin McCarthy (R-Calif.) is set to have a talk with Greene about her vile new antics. CNN reports that Greene “liked” a social media post that suggested “a bullet to the head” for House Speaker Nancy Pelosi (D-Calif.) and seemed to approve of a suggestion that other prominent Democrats should be hanged.

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There is a long-running debate among historians and political scientists about the true nature of the far-right fringe’s relationship to the GOP and the conservative movement.

In “The Long New Right,” political scientists Daniel Schlozman and Sam Rosenfeld argue that the GOP and conservative movement have allowed the boundary between fringe and mainstream to remain “porous” going back through Joe McCarthy’s anti-communist crusades in the 1950s.

That lapse, according to this thesis, is grounded in a fundamental feature of the post-war right wing, its constant addiction to a “politics of conflict” that lacks any “sense of limits, whether tactical or substantive.”

The result: The GOP and conservative movement have failed at “policing boundaries against extremism,” which defined a “half century of Republican politics.”

Examples include conservative movement leaders flirting with the John Birch Society; allies of 1964 presidential candidate Barry Goldwater refusing to disavow a Ku Klux Klan endorsement; Newt Gingrich’s conversion of GOP politics into nationalized scorched earth warfare; and, of course, the rise of Trump.

Geoffrey Kabaservice, the author of a history of the modern GOP, argued to me that GOP and conservative “gatekeepers” have gradually stood down over time, bringing us to the present.