David Horowitz, a radical leftist of the 1960s who did a political about-face to become an outspoken conservative author and activist, writing that Barack Obama had “betrayed” America, and an ardent cheerleader for Donald J. Trump, died on Tuesday. He was 86.

The David Horowitz Freedom Center, a think tank he founded in Southern California, said the cause was cancer. His wife, April Horowitz, said he died at his home in Colorado.

Once a self-described Marxist, Mr. Horowitz executed a dizzying transit from the extreme left to the extreme right. He argued that the Black Lives Matter movement had fueled racial hatred; he opposed Palestinian rights; he denounced the news media and universities as tools of the left; and he falsely claimed that Mr. Trump had won the 2020 election, which Mr. Horowitz called “the greatest political crime” in American history.

A prolific author since his early 20s, Mr. Horowitz published several pro-Trump books, including “Big Agenda: President Trump’s Plan to Save America” (2017) and “The Enemy Within: How a Totalitarian Movement Is Destroying America” (2021). The enemies he accused of totalitarian impulses were the mainstream Democrats Nancy Pelosi, then the House speaker, and Kamala Harris, then the vice president.

Mr. Horowitz was a mentor to Stephen Miller, Mr. Trump’s top domestic policy adviser, whom he met when Mr. Miller was a California high school student fervidly critical of multiculturalism.

At Duke University, Mr. Miller started a chapter of Students for Academic Freedom, a grass-roots advocacy group founded by Mr. Horowitz. Mr. Horowitz asked him to help coordinate an “Islamo-Fascism Awareness Week” on college campuses, according to Jean Guerrero, a biographer of Mr. Miller, writing in Politico in 2020.