For 48 hours his campaign had been in full meltdown mode.

Donald Trump’s response? To let Trump be Trump — and then some.

There was, of course, the tape heard round the world — the one with him bragging that he can “grab” women “by the p****” simply because he’s “a star.”

There were the unprecedented defections: more than a dozen sitting Republican senators and scores of other GOP officials tripping over each other to declare they could no longer in good conscience support his candidacy.

There was the defiant “apology” video; the conservative pleas for a new nominee; the weekend spent hiding out, huddling with friends and family, in his namesake Manhattan tower.

By the time Trump sauntered onstage at Washington University Sunday night for the second debate of the 2016 presidential election — his odds of winning the White House dwindling, his remaining options unclear — pretty much everyone in America was asking the same question:

Which Donald Trump would show up in St. Louis?

Minutes later the answer was obvious: the Trumpiest Trump ever. “I pledge to be a better man tomorrow,” Trump had said in his apology video. But during the debate he wasn’t a better man. He was the same man, only more so.

Slideshow: Body language: Hillary Clinton and Donald Trump face off for their second fiery debate >>>

This was the Trump that the alt-right website Breitbart had championed all cycle. The Trump that the loyalists who spent the weekend at Trump Tower encouraged him to be. The Trump that your neighbor with the “Hillary for Prison” T-shirt keeps posting about on Facebook.

This Trump went to 11.

Donald Trump during the second presidential debate, at Washington University in St. Louis on Sunday. (Photo: Saul Loeb/Pool via AP)

Never before has a presidential nominee of either party behaved on a debate stage the way Trump behaved Sunday night. Much of what he said would have seemed inconceivable in past presidential forums.

In a direct breach of the tradition of nonpartisan rule of law, for instance, Trump promised that, if elected president, he would appoint a special prosecutor to go after Hillary Clinton.

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Clinton tried to laugh it off. “It’s just awfully good,” she said, “that someone with the temperament of Donald Trump is not in charge of the law in our country.”

“Because you’d be in jail,” Trump snapped — seemingly unaware that, after a democratic election, the winner doesn’t imprison his political opponents.

Trump went on to liken Clinton to “the devil.” He said, “She has tremendous hate in her heart.” “We have a divided nation,” he added, “because [of] people like her.” When talk turned to Capt. Humayun Khan, the decorated Muslim American soldier who died in Iraq and whose parents have spoken out against Trump, the Republican blamed his death on Clinton, claiming that “if I were president at that time, he would be alive today, because unlike her … I would not have had our people in Iraq.” (Fact checkers have shown that Trump initially supported the war.)

Instead of displaying sincere remorse for his lewd remarks about women — “this is locker-room talk,” he repeated five times — Trump decided to play a political version of “I am rubber, you are glue,” summoning several women who had accused Bill Clinton of sexual misconduct to a surprise pre-debate press conference and immediately pivoting to the former president whenever moderators Anderson Cooper and Martha Raddatz asked about the “Access Hollywood” tape.

“If you look at Bill Clinton, far worse,” Trump said. “There’s never been anybody in the history of politics in this nation that’s been so abusive to women.”

In fact, Trump seemed to conclude, the whole firestorm was somehow his opponent’s fault. “When Hillary brings up a point like that and she talks about words that I said 11 years ago, I think it’s disgraceful,” he said. “I think she should be ashamed of herself.”

Photo: Shannon Stapleton/Reuters

Whenever Trump had a choice — dial it down or dial it up — he chose the latter. Despite charges that he has been cozying up to Vladimir Putin, he defended Russia throughout the debate, at one point going so far as to throw his running mate, Indiana Gov. Mike Pence, under the proverbial bus for suggesting that Putin’s provocations in Syria need to be “met with American strength.”

“He and I haven’t spoken,” Trump said. “And I disagree.”

Trump frequently whined to the moderators for favoring Clinton — a favorite complaint of conservatives everywhere. Of his late-night tweet storms — which some critics see as impulsive and unpresidential — Trump would only say, “I’m not un-proud of it, to be honest with you.” And when pressed by Cooper to reveal once and for all whether he used a $916 million loss to avoid paying personal federal income taxes since 1995, Trump brazenly admitted that he did.

“Of course I do,” he said. “Of course I do.”

When the dust settled Sunday night, most headlines described the debate as “vicious” — and many blamed both candidates equally. “Clinton and Trump got vicious during tonight’s debate,” read an alert from the official Washington Post account. “He threatened to send her to jail; she attacked his treatment of women.”

To be clear, though: Threatening to undermine the rule of law is not the same thing as criticizing your opponent for saying he likes to grab women by their genitalia.

Quoting Michelle Obama, Clinton telegraphed her strategy early on.

“When they go low,” she said, “you go high.”

To be sure, Clinton took her shots at Trump for the tape. “I think it’s clear to anyone who heard it that it represents exactly who he is,” she said. “Because we’ve seen this throughout the campaign. We have seen him insult women. We’ve seen him rate women on their appearance, ranking them from one to 10. We’ve seen him embarrass women on TV and on Twitter. … The question for us, the question our country must answer, is that this is not who we are.”

But then, having clearly decided it was Trump, not she, who needed to change the trajectory of the campaign, Clinton sat back, played it safe and let him attempt to do just that.

The commenters on Breitbart will be pleased with the path Trump chose. So will the 35 percent of voters who will back him no matter what.

But the rest of the country — including much of the Republican Party — may not be so charmed.

Photo: Shannon Stapleton/Reuters

In truth, Trump’s options were limited. Clinton was already well ahead in key battleground states before the “Access Hollywood” tape came out, and his chances of convincing college-educated female swing voters to change sides collapsed after its release. So why not rally the base? With the GOP establishment ditching him in droves, the die-hards are pretty much all he has left.

The problem is that there aren’t enough people in Trump’s base to win a presidential election. As the debate itself fades from memory, and as the post-debate spin cycle begins, the Clinton campaign will continue to weaponize key moments on social media and elsewhere — an art it perfected last month with Alicia Machado. It certainly came away from Sunday’s encounter with plenty of ammo: the jail comment, the tax confession, Trump’s habit of looming behind Clinton every time she spoke and so on.

Meanwhile, the GOP is facing its own conundrum. With Trump now toxic among swing voters — and refusing, it seems, to reach out to them in the final weeks of the campaign — down-ballot Republicans have an impossible decision to make. Do they repudiate Trump and move to the center, thereby risking the support of Trump’s reinvigorated base? (See Kelly Ayotte in New Hampshire and Joe Heck in Nevada.)

Or do they stick with an emboldened Trump and his ever-defiant fans — and go down with what seems to be a sinking ship?

After St. Louis, the battle between Trump and his party may actually be more interesting than the battle between Trump and Clinton.