President Trump speaks at the White House during the swearing-in ceremony for Supreme Court Justice Neil Gorsuch. (Eric Thayer/Getty Images)

Maybe we should muzzle the wag-the-dog talk.

MSNBC host Lawrence O’Donnell led off his show Friday night with an alarming report: Russian President Vladimir Putin may have told Syria’s Bashar al-Assad to launch last week’s chemical attackto let President Trump respond militarily — thereby boosting Trump’s standing in the United States and dispelling the belief that he is too close to Putin.

“It’s perfect,” O’Donnell said, telling viewers “what you won’t hear is proof that that scenario that I have just outlined is impossible, because . . . with Donald Trump anything is possible.”

I’m a fan of O’Donnell, and it is technically true that we can’t prove that Putin didn’t orchestrate the attack to boost Trump. But by that logic, we can never prove to everybody’s satisfaction that there wasn’t a second gunman on the grassy knoll, that Vincent Foster wasn’t murdered, that there wasn’t a controlled demolition inside Building 7, that former president Barack Obama didn’t forge his birth certificate, or that the government isn’t controlling our minds with fluoride.

But speculation without evidence is at best distraction, and at worst it allows Trump’s defenders to discredit the whole story about Trump’s contacts with Russia and Russia’s attempts to tilt the election his way.

(Monica Akhtar/The Washington Post)

Certainly, Trump’s behavior has shown that he’s capable of anything. But we don’t need to speculate. Putin did conspire to help Trump win the presidency. That’s damning enough without letting allegations of a chemical-attack conspiracy cloud the whole thing in paranoia.

The chemical conspiracy, as The Post’s Avi Selk noted, debuted on a left-wing site called the Palmer Report. This is part of a larger phenomenon that has already taken root online, where in some quarters full-blown cases of Trump Derangement Syndrome have already broken out. Trump won the presidency and now governs by creating a parallel universe with alternative facts. There’s a temptation among his opponents to respond in kind. But the way to counter Trump is to speak the truth, not to fight him with more fake news.

[MSNBC host’s conspiracy theory: What if Putin planned the Syrian chemical attack to help Trump?]

Trump’s campaign, transition and nascent presidency have generated liberal conspiracy theories worthy of Glenn Beck and Alex Jones. Check these days with Snopes, the conspiracy-busting website, and, right there alongside the usual urban legends (“woman arrested for training squirrels to attack her ex-boyfriend,” “female mortuary worker was arrested after becoming pregnant by one of the corpses she was preparing for burial”), you’ll find them confronting various products of the vast left-wing conspiracy:

“Senior White House officials ‘openly admitted that the chemical strike against Syria had no actual purpose.’ ”

“Donald Trump has vowed to reinstate the draft.”

“Evangelical leader Kenneth Copeland said opponents of President Trump could be ‘punished’ with gay children.”

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“Devin Nunes’ financial wealth is invested in a wine company with ‘strong ties’ to Russia.”

“Attorney General Jeff Sessions is to be disbarred from the Alabama State Bar Association due to a letter of complaint filed by 2,000 attorneys from across the U.S.”

“All eight Supreme Court justices penned a ‘letter’ saying President Trump was ‘completely wrong in choosing Neil Gorsuch’ to fill a vacant seat on the court.”

Rep. Trey Gowdy (R-S.C.) “ordered the exhumation of the body of Clinton associate Vince Foster.”

Some have gained considerable traction, such as Google engineer Yonatan Zunger’s late January post on Medium that the travel ban was “the trial balloon for a coup d’état.” As BuzzFeed noted, Huffington Post contributor Alex Mohajer built a case that Trump was involved in the $11 billion sale of Russian oil giant Rosneft. One well-subscribed theory has it that Trump’s early filing for reelection in 2020 was actually a conspiracy to silence his critics.

Robert Reich, the former Clinton cabinet member, has detected a “Trump plot to control American universities” in conjunction with chief strategist Steven K. Bannon and former Breitbart News provocateur Milos Yiannopoulos. And a dubious Twitter account claiming to be anti-Trump government officials, “@RoguePOTUSStaff” has amassed 850,000 followers by tweeting unsubstantiated news about the White House.

Brendan Nyhan, a Dartmouth government professor, explained in the New York Times in February why left-wing conspiracy theories appear to have gained since the election: “Political psychology research suggests that losing political control can make people more vulnerable to misinformation and conspiracy theories.” How else could people have fallen for the satirical report in a British outlet alleging that Queen Elizabeth II said she can legally kill Trump with a sword if he enters Buckingham Palace?

That would indeed be newsworthy, if true. But here’s something even more newsworthy: The Putin regime meddled in U.S. elections to help secure the victory of a president to whom it has had extensive ties. And that one happens to be true.

Twitter: @Milbank

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