Once a Marxist, he came to embrace hard-right positions, including the falsehood that Mr. Trump won in 2020, and to mentor Stephen Miller, later the Trump adviser.

David Horowitz, the conservative author and activist, died on Tuesday at the age of 86, as confirmed by the David Horowitz Freedom Center, the think tank he established. The center announced on X that Horowitz died following a prolonged battle with cancer.

David Horowitz, a former 1960s radical and far-left Marxist who later switched sides to become one of conservatism's greatest cheerleaders and influencers, is gone. Horowitz died of cancer Tuesday at age 86 in his home in Colorado,