When Donald J. Trump promised to turn Muslims away from American shores, they wagged their fingers.

When he mimicked a journalist for his lifelong disability, they tsked-tsked.

When he mocked the mother of a valorous soldier killed in combat, they threatened to walk away from him.

But Republican leaders never did. They justified his behavior, they minimized his offenses, they excused his insults.

So why this?

Why did a decade-old three-minute video provoke a sudden revolt by party officials against their nominee, an uprising that could very well destroy their chances of taking the White House?

Because the glee with which he bragged about sexually assaulting women, by forcibly kissing them and grabbing their genitals, turned a boorish man into an outright predator.

Because the voice captured on a microphone and the face caught on camera are indisputably Mr. Trump’s, breaking through to a distrustful public that doubts much news media coverage but believes powerfully in what it can see and hear for itself.

Because it turns out that even the most self-interested members of the political class, the true weather vanes swinging in the wind, have their limits.

After 16 months of accumulated doubts, embarrassments and indignities, they are finally fed up.

As senators, representatives and party elders rescinded their endorsements of Mr. Trump in unrelenting waves on Saturday, those who had stubbornly stuck by their nominee described their rejection in almost cathartic terms, saying he had surrendered any legitimate claim to their loyalty.

“He has forfeited the right to be our party’s nominee” was how Senator Lisa Murkowski of Alaska put it, saying she would not vote for Mr. Trump.

It did not repel everyone: Speaker Paul D. Ryan and Senator Mitch McConnell did not waver from their endorsement of Mr. Trump.

But the image of Republicans running for the exits, a month before a presidential election, is as extraordinary as a party’s nominee using vulgar, violent language that seemed to reduce an entire gender to sexual anatomy. And this time, no amount of spin seems sufficient to control the damage Mr. Trump has wrought.

When he crossed the line in the past, there was always a ready escape hatch, a set of rationalizations offered by his slack-jawed, but ultimately accommodating, Republican allies.

He had, they said, erred in his excessive zeal to protect the country’s borders, its workers or its safety (by calling Mexican immigrants rapists or proposing to bar Muslims from the United States). Or he had, they said, simply returned a provocation with a counterpunch (by ridiculing Carly Fiorina’s face as unpresidential or skewering Rosie O’Donnell for her weight).

But aboard the bus with the Hollywood flatterer Billy Bush, those strained arguments collapsed. There was no aggressor to fight back. There was no larger issue at stake.

There was just lasciviousness in its purest and cruelest form.

“It was just this unprovoked view of women by this 59-year-old man with daughters,” said Russ Schriefer, a longtime Republican strategist and ad maker who has spent his career studying the effect of images and words in campaigns.

“It becomes,” he said, “indefensible.”

Authenticity is Mr. Trump’s brand. But the politically destructive force of the video lay in its power to pull back the curtain on his true self.

Surrounded by sycophants on a luxury bus, Mr. Trump brags about how he aggressively tried to seduce a married woman, later ridiculing her figure as ruined by fake breasts.

He then spies an attractive actress waiting for him outside the bus and regales his companions with his ability to force himself on women sexually because of his celebrity.

“When you’re a star, they let you do it,” he said.

After salivating over the sight of the actress, Mr. Trump descends from the bus and acts like a gentleman. It is a moment of fraudulence that resonates deeply for any women or men who fear what people might say when their backs are turned.

“It’s Trump behind closed doors, in a candid moment in a nonpolitical setting, and this is the true inner Donald Trump you are hearing,” said Tad Devine, a veteran adviser and ad maker for Democratic presidential candidates.

It is difficult to overstate the visceral power of the recording, which left no ambiguity about the coarseness of the words or the identity of the man who had uttered them.

“He can’t say, ‘It’s not what you think it is,’ or, ‘It’s been doctored,’” Mr. Schriefer said. “It happened.”

Four years ago, Democrats spent tens of millions of dollars making the case that Mitt Romney was an unfeeling, out-of-touch plutocrat.

But it was a grainy, secretly recorded video of him dismissing 47 percent of the country as dependent on the government that ultimately sliced through the electorate and badly undermined his campaign.

“This is Romney’s 47 percent video times 10,” Mr. Devine said of Mr. Trump’s remarks.

Mr. Trump’s churlish behavior toward women is well documented, including in a 4,700-word investigation by The New York Times.

As a boss, he told female employees that they needed to lose weight. As a boyfriend, he asked women to rate his previous romantic partners on a scale of one to 10, one of them recalled.

As a father, he marveled at his daughter’s body. “She’s hot, right?” a contestant in one of his beauty pageants recalled him saying. And as a husband, he was accused of rape by his first wife, Ivana, though she later backed away from that claim.