Michael Gerson, syndicated columnist and former aide to President George W. Bush, explained in The Washington Post that presumptive GOP presidential nominee Donald Trump’s candidacy is “fueled by conspiracy.”

Trump has peddled numerous conspiracy theories, including leading the charge in questioning the validity of President Obama’s birth certificate, and claiming vaccines cause autism, that the government lied about the dangers of Ebola, that Muslims cheered on 9/11, and that Supreme Court Justice Antonin Scalia may have been murdered, among others. Trump regularly surrounds himself with and lauds known conspiracy theorists like Alex Jones, an infamous 9/11 truther, and Roger Stone, a notorious dirty trickster who alleges the Clintons are murderers.

In a May 23 opinion piece for the Post, Gerson wrote Trump “is not flirting with the fringes” by pushing “conspiratorial nonsense,” but “is French-kissing them.” Gerson explained that “Trump emerged in conservative circles by questioning Barack Obama’s citizenship, and thus the legality of his presidency,” and has since peddled numerous conspiracy theories and has “succeeded by appealing to stereotypes and ugly hatreds.” Gerson warned “every Republican official endorsing Trump” that the conspiratorial “company he keeps … is the company you now keep.” From the May 23 Washington Post opinion piece:

But it was Donald Trump who led the opposition. He tweeted: “The U.S. must immediately stop all flights from EBOLA infected countries or the plague will start and spread inside our ‘borders.’ Act fast!” And: “Ebola is much easier to transmit than the CDC and government representatives are admitting.”

Health officials were not lying. Travel to and from West Africa was essential for medical personnel and aid workers to defeat the disease at its point of origin. Trump’s ban would have made Ebola materially more likely to spread beyond control.

What kind of politics is ascendant in the United States? A distrust of institutions that borders on conspiratorial. Here is Trump again: “Healthy young child goes to doctor, gets pumped with massive shot of many vaccines, doesn’t feel good and changes — AUTISM.” And: “I am being proven right about massive vaccinations — the doctors lied.” And: “So many people who have children with autism have thanked me — amazing response. They know far better than fudged up reports!”

Lying doctors. Fudged reports. It would all be disturbing — if it were not conspiratorial nonsense. No connection has ever been demonstrated between vaccinations and autism. And this particular nonsense is potentially deadly. Trump is undermining a consensus for vaccination that builds up “herd immunity” and saves the lives of children.

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Does Trump really believe that liberals may have ordered a hit on a Supreme Court justice? Who knows? We do know he finds such ideas useful. Trump emerged in conservative circles by questioning Barack Obama’s citizenship, and thus the legality of his presidency. This required the existence of a conspiracy to hide the circumstances of Obama’s birth. “They cannot believe what they’re finding,” he said of “people that have been studying it.” Having actually discovered nothing, Trump doubled down on a deception.

As a leader, Trump has succeeded by appealing to stereotypes and ugly hatreds that most American leaders have struggled to repress and contain. His political universe consists of deceptive experts, of scheming, of criminal Mexicans, of lying politicians and bureaucrats and of disloyal Muslims. Asked to repudiate David Duke and the Ku Klux Klan, Trump hesitated, later claiming a “bad earpiece.” Asked to repudiate the vicious anti-Semitism of some of his followers, Trump responded, “I don’t have a message to the fans.” Wouldn’t want to offend “the fans.”

This is not flirting with the fringes; it is French-kissing them. Every Republican official endorsing Trump should know: This is the company he keeps. This is the company you now keep.