What happened Saturday in American politics was profound, and for Donald Trump, it marks the likely death of his presidential campaign. The permission structure that was slowly, reluctantly built around Trump by the media, by the Republican Party, by the conservative movement, and by Trump himself is crumbling, and there is no time to rebuild it before the election.

Since Trump won the GOP primary, his party has been working to normalize an abnormal, unpopular, and frankly bizarre candidate. Trump has always had the support of his hardcore fans — but he needed more than that. He needed the mainstream Republicans who didn’t vote in the GOP primaries, or voted for someone else; he needed the independents who didn’t like Clinton, but didn’t trust Trump; he needed the movement conservatives who had entered politics to ban abortion and shrink the government, and who saw little of their crusade reflected in Trump’s history.

Trump needed, to put it simply, the support of a lot of voters who didn’t like him.

And so the party began to make its pitch. Mitch McConnell endorsed Donald Trump, and so, eventually, did Paul Ryan. Fox News ended its brief feud with the Republican nominee and closed ranks around his campaign. Reince Priebus dispatched top RNC staffers to work for Trump’s campaign. Marco Rubio offered his support. The messaging was simple, and sober: better Trump than Hillary, better the lunatic who will nominate our judges and sign our tax bills than the liberal who won’t, and even if you don’t believe in any of that, better to support Trump and keep the House then to abandon him and witness the return of Speaker Pelosi.

The process took on its own momentum. To be a Republican in good standing, in recent months, has meant accepting Trump as the party’s nominee, and working to see him elected. As more Republicans backed Trump, the rest had to follow suit. John McCain endorsed Trump, even though Trump had mocked his war heroism. Ted Cruz endorsed Trump, even though Trump tied his father to JFK’s assassination and mocked his wife’s looks.

The effect of all this has been to build a floor under Trump’s vote share and his media coverage. He might lose the election, but so long as Republicans were able to signal that he’s the guy you vote for if you don’t want Hillary Clinton, he couldn’t lose it that badly — and a 3- or 4-point loss would probably mean Republicans kept the House and Senate. By the same token, Trump gets plenty of bad press, but so long as the Republican Party stood behind him, he had to be covered as a basically normal candidate, not as a dangerous virus that had somehow infected American politics.

Trump’s army of validators protected him from feeling the force of his own abnormality, of his bizarre comments, of his frequent gaffes, of his erratic behavior. It was a permission structure for voters who didn’t like Trump to vote for him anyway, and it worked. Polls routinely showed that the number of people supporting Trump was larger than the number of people who thought him qualified for the presidency. Voting for Trump, in this context, became what you did if you were a Republican, or if you didn’t like Hillary Clinton. Why? Well, that’s what everyone else like you was doing, too.

And now that’s crumbling. The wave of Republican defections from Trump’s side is the same process that normalized him, but in reverse. Just as Republicans felt more and more pressure to support his candidacy as other Republicans signed on, Republicans — and everyone else — will feel more and more pressure to abandon his candidacy as their co-partisans defect.

The Trump defectors span the gamut. There’s Sen. Kelly Ayotte, who is in a tough reelection race in New Hampshire; there’s Sen. Mike Crapo, who is cruising to victory in Idaho; there’s Rep. Jason Chaffetz, a rising star congressman from Utah; there’s Rob Portman, the popular Republican senator from the crucial-for-Trump state of Ohio; and dozens more.

But it is McCain’s statement that drives the knife deepest. His support had served as a kind of signal that even those who’d been brutally attacked by Trump had to back him for the good of the country, and his abandonment speaks directly to that impulse. "I have wanted to support the candidate our party nominated,” he said. “But Donald Trump's behavior this week, concluding with the disclosure of his demeaning comments about women and his boasts about sexual assaults, make it impossible to continue to offer even conditional support for his candidacy."

McCain speaks not for those who enthusiastically supported Trump but for those who did it grudgingly, conditionally, out of a sense of duty to their country. That same sense of duty, he is now saying, forces decent men and women to abandon Trump’s campaign.

It’s not just Trump’s backers who are defecting. His enemies are emboldened, and party elders who stayed neutral are now off the sidelines. “Enough!” wrote Condoleezza Rice on Facebook. “Donald Trump should not be President. He should withdraw. As a Republican, I hope to support someone who has the dignity and stature to run for the highest office in the greatest democracy on earth.”

The signals here are clear and unmistakable. As Max Fisher noted on Twitter, “Real coups usually begin with a wave of public leaks; institutional signaling that support has shifted and defectors will not be punished.” This is what is happening to Trump right now: A key class of Republicans are signaling to each other and to everyone else that history has shifted and defectors will not be punished — in fact, they will be celebrated.

The signal will reach ordinary voters, too. If even all these Republicans are abandoning Trump, then what does it say about you if you vote for him? If both sides agree that Trump is too loathsome, erratic, and cruel to deserve the presidency, then do you really want to be his supporter?

Two weeks ago, supporting Trump was what you did if you were a Republican. Today, abandoning Trump is what you do if you’re a decent person. Two weeks ago, Trump’s floor was probably the mid-40s. Now it is plausibly much lower — I would no longer be surprised to see Trump’s vote share dip down into the 30s, with disastrous results for the Republicans congressional majority.

Trump is becoming de-normalized, and the structure that made it safe for Trump skeptics to vote for him is crumbling. That is a very dangerous dynamic for the Republican nominee, and for the political party that tried for so long to protect him.