In the early 1960s, a dedicated group of extremists coalesced around a series of anti-communist conspiracy theories.

Perhaps the most famous was the claim made by Robert Welch Jr., the founder of the John Birch Society, who told an audience in 1961 that Dwight Eisenhower is “a conscious agent of the Communist conspiracy.”

Yes, that Eisenhower, the U.S. Army general, the hero of World War II and the two-term president of the United States.

Conservative Republicans of that era — even those who were rabidly anti-communist — put distance between radical Birchers and themselves. Ike a commie? Anyone willing to voice such nuttiness was relegated to the fringes of U.S. politics.

That was then. This era’s lunatic theory is that President Barack Obama was not born in the United States. The difference between now and 55 years ago is that a unified movement on the right dedicated to knocking down birtherism never took hold.

Thus, Donald Trump became the 2016 Republican nominee for president despite expressing loud and public doubts that Obama was born in the United States. Only last week did Trump admit the truth about Obama’s birth. Even then he tried to shift blame to his Democratic rival, Hillary Clinton, saying she and her campaign were the original birthers. That’s not true.

This week, Trump was back to the topic, telling a TV interviewer he flip-flopped on birtherism only so he could “get on with the campaign.”

There’s already plenty of evidence that testifies against Trump’s qualifications to be president. Yet, he is the Republican nominee. How a major political party was captured by someone spewing this birther nonsense is deeply troubling.