Assembled in one place, the details of Moscow’s deception are more startling than ever. Fraudulent news stories planted by paid trolls on Facebook often outperformed those from legitimate outlets. The top two in the three months leading up to the election featured Pope Francis endorsing Mr. Trump and Ms. Clinton selling weapons to the Islamic State. The Kremlin would often start with truthful or otherwise innocuous content and then deliver what it called payload posts. “Rise and shine — realize His blessing!” on a page called “Army of Jesus” preceded the missive “HILLARY APPROVES REMOVAL OF GOD FROM THE PLEDGE OF ALLEGIANCE.”

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The Russians extended their reach beyond the online realm with rallies and spontaneous gatherings known as flash mobs, at one point attempting to draw a crowd in New York City by offering free hot dogs and at another paying participants to portray Ms. Clinton imprisoned in a cage. A protest against Islam in Houston by a made-up group called “Heart of Texas” and a manufactured pro-Muslim event held at the same time resulted in a confrontation that local news agencies covered live. The operation cost operatives in St. Petersburg only $200.

“The preponderance of the operational focus,” write the Senate authors, “was on socially divisive issues . . . in an attempt to pit Americans against one another and their government.” The Kremlin’s hope throughout its history of disinformation operations has been to turn the public “passive and paranoid,” convince people they cannot rely on traditional channels for the truth, and lure governments into responding in a manner “irreconcilable with the nation’s principles and civil liberties.”

All of this is eerily familiar. The president today is spreading conspiracy theories like birdseed, suggesting counter to all available evidence that it was Ukraine that attempted to intervene in the election — and that it did so to aid his rival. Worse still, he is inviting foreign nations to meddle some more. And the same Republican representatives and senators whose colleagues crafted this impressive report are meekly standing by, even as the White House refuses to cooperate with a congressional inquiry designed to get to the truth.

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The report recommends that politicians be cautious in the information they choose to share. After all, they are rich targets for adversaries to trick into “inadvertently promoting” their anti-democratic aims. In Mr. Trump’s case, “inadvertent” seems a generous adjective.