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Donald Trump and Hillary Clinton are waging the nastiest debate in presidential history.

The tension was on full display from the opening moment of the debate Sunday night when they took the debate stage and -- in a departure from tradition -- didn't shake each other's hands.

Trump repeatedly said the vulgar language he used about women more than a decade ago was just "locker room talk" before pivoting to accusations about Bill Clinton, calling the former president "abusive."

The stunning developments come as Trump tries to shake off the fallout of the release of a decade-old video tape that shows him making lewd and sexually aggressive remarks about women. Trump insisted he had not approached women without their consent but said he was "embarrassed" by the tape. He constantly tried to redivert the conversation even when challenged by CNN's Anderson Cooper to his plans to "Make America Great Again," saying he would go hard after ISIS for example.

Clinton said she had been thinking very carefully about the tape.

"Yes, this is who Donald Trump is," Clinton said. "This is who Donald Trump is but the question our country must answer is this is not who we are."

She said that she had differed before with Republicans over policy.

"I never questioned their fitness to serve," she said. "Donald Trump is different."

Hours earlier, Trump made a surprise appearance with women who have in the past accused Bill Clinton of inappropriate sexual activity. Trump spokesman Jason Miller said the women will be in the audience at the debate.

Dashing hopes of GOP establishment

Trump's pre-debate surprise likely dashes hopes of Republican establishment leaders who spent the weekend arguing the nominee would be best served if he could turn away from his more volatile instincts. The turmoil in Trump's campaign have tipped the Republican Party into chaos. Dozens of elected Republicans in Washington and state capitals around the country have condemned Trump -- and many have called for him to step aside. His own running mate, Mike Pence, said Saturday he can't defend Trump's comments.

President Barack Obama joined in the condemnation Sunday, saying the comments suggest Trump "doesn't care much about the basic values."

"One of the most disturbing things about this election is just the unbelievable rhetoric coming at the top of the Republican ticket," Obama said at a campaign event in Chicago for US Senate candidate Tammy Duckworth. "I don't need to repeat it. There are children in the room."

Meanwhile, Clinton has largely stayed quiet, letting the turmoil in the GOP unfold and preparing to address the controversy publicly for the first time before an audience of millions.

Trump heads into the debate under intense pressure to deliver a powerful performance that demonstrates regret and remorse for his conduct. Only a top-notch showing will help Trump stem the GOP exodus away from his campaign that kicked into full gear Saturday.

On Sunday, he struck back, hinting to Republicans distancing themselves from him that they would regret it.

"So many self-righteous hypocrites. Watch their poll numbers - and elections - go down!" he tweeted.

And his top Sunday show surrogate, Rudy Giuliani, tried to deflect concerns while showing contrition.

"The fact is that men at times talk like that. Not all men, but men do. He was wrong for doing it. I am not justifying it. I believe it's wrong. I know he believes it's wrong. I believe this is not the man we're talking about today," Giuliani told CNN's Jake Tapper on "State of the Union."

Knockout punch for Clinton?

Clinton will enter the showdown sensing she might be able to land a knockout punch that finally kills off Trump's campaign at its moment of greatest vulnerability, less than a month before voters go to the polls. But she will be on guard for any risky gambit by Trump to try to deflect from the uproar surrounding his lascivious past by bringing up allegations of sexual misconduct by her husband, former President Bill Clinton, in the 1980s and 1990s.

The drama will unfold before tens of millions of viewers and will be a fresh reminder how the former star of "The Apprentice" has effectively turned American politics in 2016 into one giant reality show revolving around himself and consuming the whole country.

Trump's attempt to rescue his campaign will be all the more complicated since the debate will use a town hall format -- a tricky one for novice candidates -- and will take place in front of a group of undecided voters, some of whom could have pointed questions about his behavior.

Trump must address female voters in particular, given that there is no path to the White House for him unless he can improve his standing among college educated suburban women in key swing states like Pennsylvania and Colorado.

His task is made even more daunting given that he botched the first debate nearly two weeks ago, crushing all the momentum he built up in September and sending his campaign into a spiral, from which, it has not recovered.

Trump's weekend troubles, which produced the most stunning 24 hours of an already rollicking 2016 campaign, forced the Clinton campaign into some last minute adjustments before the debate.

The former secretary of state spent Saturday retooling her strategy. Sunday's showdown offers her a chance to hit Trump when he is down and to deliver a closing argument about why his current troubles validate the central theme of her campaign, that he lacks the temperament and character required of a president in the Oval Office.

"I think what she needs to is come out fierce, come out and condemn Donald Trump in the strongest words she has ever used," Patti Solis Doyle, who ran Clinton's 2008 primary campaign, said Saturday on CNN. "She needs to stand up for these women that he has insulted, that he has offended."

Palmieri told reporters Sunday that the candidate had been briefed on Trump's comments during debate prep but couldn't confirm that Clinton had actually watched the video.

"We briefed her on it during debate prep," Palmieri said. "So many memorable moments in this campaign that they no longer become memorable. This is one of them."

As well as going on the offensive against Trump, Clinton is also likely to take advantage of Sunday's massive television audience to lay out some of the policy approaches she would adopt to improve the lives of everyday Americans.

But all eyes will be on how Trump responds to the crisis enveloping his campaign. His attempt to save his White House hopes with a big television moment has few precedents in modern political history.

Ironically, one parallel might involve Clinton herself. The future first lady appeared on "60 Minutes" with her husband in 1992 and turned in a stunning performance to rescue his Democratic primary campaign and political career after he was assailed by allegations of infidelity.

Forty years earlier, Richard Nixon pulled off the same kind of high wire act on television that faces Trump when he was in danger of being thrown off Dwight Eisenhower's presidential ticket due to a campaign finance scandal. His appearance, which came to be known as the Checkers speech, was good enough for him to be retained as vice presidential nominee and saved his political future.

Trump needs nothing less on Sunday.