A main cause of the rise of extremism in the world of Islam has been the cowardice of Muslim moderates, who for decades chose not to condemn bad ideas and ugly rhetoric. Fearing that they’d be seen as ideological weaklings, they’ve avoided confronting the cancer in plain sight. It is now clear that a similar dynamic has been at play in the world of conservatism.

Mitt Romney should be congratulated for making a speech calling Donald Trump a phony and a fraud. But where was he in 2012, when Trump was pushing his nasty — and utterly false — campaign casting doubt on President Obama’s U.S. citizenship?

By Trump’s side in Las Vegas, as E.J. Dionne Jr. reminds us in his book “Why the Right Went Wrong.” “There are some things that you just can’t imagine happening in your life,” Romney gushed. “Having his endorsement is a delight. I am so honored and pleased.” And although he generally eschewed “birtherism,” Romney fed the fires later that year by joking that “no one’s ever asked to see my birth certificate.”

There have always been radicals on both sides of the political spectrum. But what is different about the conservative movement is that, since the 1990s, some of its most distinguished mainstream members have embraced the rhetoric and tactics of the extremes. A memo put out by Newt Gingrich’s political action committee that decade urged Republican candidates to use savage rhetoric against their Democratic opponents. Some of the recommended words were “failure,” “pathetic,” “disgrace” and “incompetent.” In the past month, Trump has called Mitt Romney a “failed candidate,” Jeb Bush “pathetic,” Sen. Lindsey Graham (R-S.C.) “a disgrace” and Obama “totally incompetent.” Perhaps he read the memo.

It is gratifying to see the National Review mobilize against Trump, decrying his “free-floating populism” and disdain for the details of public policy. But where were the magazine’s editors when Sarah Palin put these same forces on full display eight years ago? Loudly cheering her on. National Review’s editor praised her for her “plain-spoken, combative way.” And he was more restrained than the editor of the Weekly Standard, William Kristol, who called Palin his “heartthrob.”

Palin knew next to nothing about national or international public policy, but she almost celebrated that ignorance, playing to the anti-intellectualism and anti-elitism of parts of the conservative base. Instead of pointing out that knowledge and expertise are actually things to admire and acquire, not mock, conservative intellectuals expressed admiration. Robert Kagan, a distinguished writer and a contributing columnist to The Post, declared: “I don’t take this elite foreign policy view that only this anointed class knows everything about the world. I’m not generally impressed that they are better judges of American foreign policy experience than those who have Palin’s experience.”

It is courageous of dozens of Republican foreign policy leaders now to sign an open letter condemning Trump publicly and refusing to support his candidacy. But over the past decade, I can recall conversations with some of these individuals in which they refused to accept that there was any problem within the Republican Party, attributing such criticism to media bias.

We still see this denial, with the truly bizarre claim by some in the media that the rise of Trump is really all the fault of . . . Obama. The logic is varied. For some, it is because he has been so weak. The Wall Street Journal editorial page opined, “The oldest truism in politics is that demagogues flourish in the absence of leadership.” (I must confess to never having heard of that “truism” and wondered how it would explain the rise of Father Charles E. Coughlin and Huey Long during Franklin Roosevelt’s reign, or Joseph McCarthy under Dwight Eisenhower.) For others, however, it is because Obama has been too strong, abusing executive power and elevating himself to center stage. Apparently having Oprah share the stage with you leads to authoritarian populism.

Here is a much simpler explanation for Donald Trump: Republicans have fed the country ideas about decline, betrayal and treason. They have encouraged the forces of anti-intellectualism, obstructionism and populism. They have flirted with bigotry and racism. Trump merely chose to unashamedly embrace all of it, saying plainly what they were hinting at for years. In doing so, he hit a jackpot.

The problem is not that Republican leaders should have begun to condemn Trump last year. It is that they should have condemned the ideas and tactics that led to his rise when they began to flourish 20 years ago.

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