People who question whether the Earth is round — a fact understood by the ancient Greeks and taught to American children in elementary school — might have been political pariahs a decade ago. Now, they’re running local Republican parties in Georgia and Minnesota and seeking public office in Alabama.

A prominent far-right activist who has said, despite years of research and intelligence establishing otherwise, that the terrorist attacks on Sept. 11, 2001, were an inside job by the U.S. government commemorated the 9/11 anniversary last year alongside President Trump.

And Robert F. Kennedy Jr., the secretary of the Department of Health and Human Services, pledged the agency’s support last month for a fight involving so-called chemtrails, a debunked theory that the white condensation lines streaming behind airplanes are toxic, or could even be used for nefarious purposes.

Conspiracy theories that were relegated to random and often anonymous online forums are now being championed

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