This is what happens when an entire political party takes leave of reality.

Recently, one of the most senior and important Republican senators used one of the most prominent settings in official Washington to level one of the most serious allegations possible against the commander in chief. And his charge was based on an utter fabrication.

Sen. Jim Risch of Idaho, ranking Republican on the Senate Foreign Relations Committee, used a hearing intended for sworn testimony from the secretary of state on the Afghanistan withdrawal to allege that President Joe Biden is mentally incompetent.

Risch first devoted his opening statement to continuing the long-running Republican narrative. “We know for a fact the president of the United States is somewhat disadvantaged here in that someone is calling the shots. He can’t even speak without someone in the White House censoring it or signing off on it,” the senator claimed. “As recently as yesterday, in mid-sentence, he was cut off by someone in the White House who makes the decision that the president of the United States is not speaking correctly. ... This is a puppeteer act.”

Then, as the first Republican questioner, Risch used his time to elaborate on the slander. “Somebody in the White House who has authority to press the button and stop the president, cut off the president’s speaking ability and sound,” he told Secretary of State Antony Blinken. “Who is that person?”

Blinken chuckled as he replied that the loose-lipped Biden “speaks very clearly and very deliberately for himself.”

Risch was unconvinced. “Are you saying there is no one in the White House that can cut him off? Because yesterday that happened and it’s happened a number of times before that.”

Smiling, Blinken assured Risch that “there is no such person.”

Risch insisted, again, that “it happened yesterday at the Interagency Fire Center. It was widely reported. ... Are you telling this committee that this does not happen, that there is no one in the White House who pushes the button and cuts him off in mid-sentence?”

After yet another back-and-forth on the matter, Blinken finally told the senator: “I really don’t know what you are referring to.”

There’s good reason for that: It didn’t happen.

The episode is worth unpacking because it shows, in miniature, how misinformation infects the Republican Party, rapidly spreads through partisan media and contaminates elected GOP leaders — who amplify and defend the falsehood, even when it’s shown to be wrong. This is how lies are born.

(In this case, it wasn’t even a useful lie. By making it his lead-off attack, Risch distracted attention from the Biden administration’s botched pullout from Afghanistan, a serious matter. Instead, Risch used the forum to portray Biden as senile, based on rubbish.)

The story begins with the White House’s recent press schedule, which announced that Biden would receive a wildfire briefing in Boise. The press coverage was listed as “out-of-town pool spray at the top.” In English, this means that a group of the traveling White House press corps is admitted to the event at the beginning and then brought out after journalists get some video and audio. It’s a routine practice presidents have used for decades. And that’s exactly what happened. According to the pool report, the group was escorted in at 12:08 p.m. for the start of the briefing and “escorted out at roughly 12:35” — a relatively long spray.

That would have been the end of the matter, except that somebody in the research department at the RNC, watching the White House’s livestream, decided that something nefarious had happened. “White House feed cuts out as Biden starts to ask a question,” the RNC tweeted.

Rupert Murdoch’s New York Post, crediting the RNC for the scoop, published an article under the headline, “White House livestream cuts Biden mid-sentence as he goes off script.”

“An official White House livestream of President Biden’s remarks was abruptly cut off mid-sentence,” it began, linking that day’s “incident” to White House officials’ “fear he’ll veer wildly off-message.”

All that was left was for Risch to trumpet the fabrication — and then to demand to know why Blinken was “unaware this is actually happening.”

Had Risch been duped, or was he deliberately parroting disinformation? I explained to Risch’s aides what had actually happened and asked whether there would be a clarification. There would not be. “Who cut off the president? Please advise,” repeated Risch spokeswoman Suzanne Wrasse.

So Risch knows the facts but perpetuates the fiction. If only somebody could “press the button” and cut off the GOP lie machine before it destroys us all.

Follow Dana Milbank on Twitter: @Milbank.