WASHINGTON—At the first presidential debate, Hillary Clinton questioned her opponent’s temperament.

And she beat him. So, naturally, he set about proving her right.

Donald Trump’s lopsided Monday defeat has sent him into a raging tailspin. Over the course of four days, in ways that seem far more vitriolic than strategic, the Republican presidential candidate has lashed out at Clinton, her husband, the media, the moderator, and a Latina former beauty queen about whom he posted a bizarre attack on Twitter before dawn on Friday.

“This is the worst post-debate spin in world history,” Rick Tyler, a MSNBC political analyst and a former spokesman for Trump rival Ted Cruz, said in an interview. “The debate was bad enough. And he just has compounded all his problems. The Clinton camp must be overjoyed. They are playing him like a fiddle.”

Tyler added: “Everybody’s been saying, ‘Is this the thing that does him in?’ The answer is: yes, this is it. He’s a joke, he’s a laughingstock, he’s a punchline. He will not recover from this.”

Trump has defied numerous previous predictions of doom. But his behaviour, including an instantly notorious Friday tweet in which he urged people to watch a “sex tape,” is also alarming allies who had been heartened by his discipline — that is, his relative approximation of discipline — in the weeks leading up to the debate.

He had surged in the polls, to a near-tie, as he did a better job sticking to something resembling a normal message. He is now in the middle of a debate swoon he is threatening to turn into a crippling post-debate fiasco.

“Trump is on the verge of blowing it,” Ari Fleischer, the former George W. Bush press secretary, wrote on Twitter. “Free advice: Focus on Hillary. No one else. Hillary is your opponent. No one else is.”

Trump has been attacking Clinton, but often in bitter ways that do not appear helpful. On Wednesday, he mocked her for being unable to make it into a car without assistance when she had pneumonia on Sept. 11. At least twice, he has noted that she failed a bar exam as a young woman — suggesting, for reasons unknown, that the person who just trounced him was lacking in intelligence.

Most of his anger, though, has been directed at the woman Clinton transparently hoped he would target: actress and former beauty queen Alicia Machado, who won the 1996 edition of Trump’s Miss Universe pageant and then was forced by him to exercise in front of cameras after she gained weight.

Clinton used precious time at the end of the debate to bring up his alleged disparagement of Machado’s weight and ethnicity. Confounding but not surprising observers, Trump leapt headfirst at the bait.

“He has an image of himself of never losing. He’s a winner. So if he’s perceived as having lost on an issue, he can’t let it go. That’s why he needs to get into conspiracy theories,” John Ziegler, a conservative talk radio host and Trump critic, said in an interview.

“It’s clearly all out of ego. He has an incredibly large ego, he’s massively insecure, he’s incredibly easy to bait. And it’s hard for me to see at this point how he turns it around,” he said. “It’s all about saving face, it’s all about saving his ego, it’s all about his own insecurities.”

Trump did not deny Clinton’s accusations about his 1990s conduct. The next morning, he repeated it: he went on Fox News and criticized Machado’s weight again. Then, on Friday morning — or, perhaps, the middle of Thursday night — it got worse.

Trump tweeted criticism of the media at 3:20 a.m. At 5:14 a.m. and 5.19 a.m, he castigated Machado. At 5:30 a.m., he wrote this: “Did Crooked Hillary help disgusting (check out sex tape and past) Alicia M become a U.S. citizen so she could use her in the debate?”

Machado does not have a “sex tape,” though she was once filmed moaning in bed with a man on a reality TV show. (There was no onscreen nudity.) There is no evidence Clinton helped Machado, a native of Venezuela, obtain citizenship.

And from a political perspective, Tyler called the attack “incomprehensible.”

“It just confirms people’s biggest concern about him,” he said. “Which is that he has the temperament of a 2-year-old.”

Clinton responded with a series of her own tweets, hers during business hours.

“This is ... unhinged, even for Trump,” she wrote. “What kind of man stays up all night to smear a woman with lies and conspiracy theories?” She added: “When something gets under Donald’s thin skin, he lashes out and can’t let go. This is dangerous for a president.”

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Polls of debate-watchers suggest Clinton won a massive victory — she was up by an average of 27 points. Post-debate state polls suggest the win has improved her overall standing considerably.

“There have been 11 post-debate polls in swing states so far and Clinton’s led in all 11. Something’s definitely changed,” FiveThirtyEight analyst Nate Silver wrote on Twitter.

Trump’s week might have been rough even if he had behaved more conventionally.

Newsweek revealed that he had spent money in Cuba in 1998, an apparent violation of the U.S. embargo. The Los Angeles Times revealed that employees have alleged he wanted to fire women he did not find sufficiently “pretty.” And the Washington Post revealed that his problem-plagued charity had not even been registered to legally solicit donations.

The next debate, a town hall, is on Oct. 9. Trump, who dismayed many female voters on Monday by interrupting Clinton dozens of times, has suggested that he plans to be even more aggressive in that one, maybe bringing up Bill Clinton’s sexual past. Ziegler called this plan “strategically suicidal.”

“You can’t attack a woman in a town hall,” he said. “All we’re going to see is pictures of horrified men and women, probably, in the studio audience, as he sinks his teeth into her. It’s going to be brutal.”

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