A former speechwriter for George W. Bush and current Washington Post columnist is criticizing conservative media figures for promoting a conspiracy theory about the death of Seth Rich, the Democratic National Committee staffer who was shot and killed last summer.

Michael Gerson, who served as Bush’s top speechwriter from 2001 through 2006, argues in a column published by the Post on Thursday that the “failure of decency” by popular right-wing personalities like Sean Hannity and Rush Limbaugh indicates deeper problems within the conservative movement.

“This is a concrete example of the mainstreaming of destructive craziness,” Gerson writes.

Rich was lethally shot near his home in Washington, D.C., in what police believe was a botched robbery. But because he was killed shortly before WikiLeaks published thousands of internal DNC emails, some conspiracy theorists (including Reddit users and right-wing bloggers) have claimed, with no evidence, that Hillary Clinton and the Democratic Party had Rich killed in retaliation for leaking the emails.

The claim picked up steam again after Fox News and its Washington affiliate published a thinly sourced story tying Rich to WikiLeaks. (The network later retracted the story.) Both Hannity and Limbaugh promoted the conspiracy following the Fox report, despite pleas from Rich’s family to stop.

Gerson argues that the movement of the story from fringe websites to cable news illustrates the way conservatism has morphed in the era of President Donald Trump ― himself a noted pusher of conspiracy theories.

“The conservative mind, in some very visible cases, has become diseased,” Gerson writes. “The movement has been seized by a kind of discrediting madness, in which conspiracy delusions figure prominently. Institutions and individuals that once served an important ideological role, providing a balance to media bias, are discrediting themselves in crucial ways. With the blessings of a president, they have abandoned the normal constraints of reason and compassion.”

This abandonment, he argues, inevitably leads conspiracy theorists to ignore the tragedy at the center of these stories — in this case, Rich and his family — and instead focus solely on the political machinations they believe are at hand.

“In Trump’s political world, this project of dehumanization is far along,” he concludes. “The future of conservatism now depends on its capacity for revulsion. And it is not at all clear whether this capacity still exists.”

Hannity, by far the most prominent proponent of the Rich theory, has faced intense backlash for pushing the story on his Fox News show, radio program and Twitter account. Multiple advertisers have pulled their ads from his Fox show, and #FireHannity has become a popular rallying cry on Twitter. Some Fox staffers also expressed their disappointment in the host to CNN, saying they were “disgusted” by his coverage of the story.

Aaron Rich, the brother of the slain DNC staffer, pleaded with Hannity in an emotional letter to drop the story.

“Nobody wants to solve Seth’s murder more than we do,” Aaron Rich wrote. “However, providing a platform to spread potentially false, damaging information will cause us additional pain, suffering and sorrow. By airing this information, you will continue to emotionally hurt us.”