The entire calculus of the strange US presidential election has shifted. At once, the election’s wild-card candidate, Republican Donald Trump, seems thrown back on his heels, and is struggling to hold together his hitherto surprisingly potent campaign.

With plunging poll numbers, Trump has two immediate crises: how to halt an exodus of establishment Republican support from him and how to beat Hillary Clinton in their crucial second debate, to be held tonight in St. Louis, Missouri.

If he can do the second, the first may take care of itself. But the bar is now higher than ever.

Trump’s poll numbers were already sinking when an outright crisis surfaced on Oct. 7. That was when the Washington Post published a 2005 tape in which Trump boasts that his stardom gives him special privileges with women—to “grab them by the pussy,” among other vile things.

The fallout is hard to explain, since Trump has made equally outrageous remarks in his almost two-year campaign for president, only to beat 17 fellow Republicans for the party nomination, and to pull even with Clinton in recent polls.

A signal that the explicit tape isn’t business as usual came a few hours after its release when Paul Ryan, speaker of the House of Representatives, disinvited Trump from a campaign event with him the next day in Wisconsin.

Trump then issued what he probably considered an apologetic video. Only, it wasn’t entirely convincing, and it didn’t help much: Only days after ridiculing a former Miss Universe on Twitter, Trump asserted in the video that he had “traveled the country talking about change for America, but my travels have also changed me.” And then he concluded, “I’ve said some foolish things, but there’s a big difference between the words and actions of other people. Bill Clinton has actually abused women, and Hillary has bullied, attacked, shamed and intimidated his victims.”

With Trump’s non-contrite contrition, a floodgate of pent-up anger opened. Actor Robert DeNiro said he was on his way to Trump Tower to “punch Donald Trump in the face.” In a more serious blow, two senior senators–widely respected John McCain, and John Thune, the third-ranking Republican in the Senate–withdrew their support for Trump.

Which brings us back to tonight’s debate. Even before the crisis, Trump had to score a decisive debate victory given that Clinton won their first face-off, and that his poll numbers are extremely low: At FiveThirtyEight, Trump is given just a 22.7% chance of winning the election.

But the tape scandal has moved the ground reality. Before the crisis, Trump seemed unlikely to unleash the nuclear option, which was an attack on Clinton based on the infidelities of her husband, former president Bill Clinton. Such an attack was seen as extremely risky because it could backfire by creating voter sympathy for Hillary Clinton.

Judging by Trump’s “apology” videotape, he now seems prepared to go nuclear regardless of the risks. And that is because his candidacy appears to be teetering: The exodus of Republicans could turn into a stampede, and he might permanently lose the votes that could close the gap with Clinton.

“Given his perilous position in the polls, an OK debate wouldn’t be enough for Trump,” said Nate Silver, editor-in-chief of FiveThirtyEight. “He needs at least a good debate, and maybe a very good debate, since it represents one of his last and best opportunities to turn the race around.”