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They are poised for another clash in the third presidential debate, which will be their final chance to change the trajectory of the election as millions of viewers tune in for the last big televised spectacular of a rancorous campaign.
Trump is under intense pressure to use the debate, just 20 days before Election Day, as a springboard for a comeback after a slide in the polls that started after Clinton's victory in the first debate and leaves him a heavy underdog.
Clinton will seek to consolidate her strong position, knowing that a winning performance could help close out Trump. But she is facing a barrage of new attacks following an avalanche of hacked emails released over the past week from WikiLeaks. She is bracing for a ferocious counter-attack from a foe who has proven in past debates that there's no line he won't cross.
One of the first questions about the debate will be answered before it even gets under way: Will the bitter rivals even shake hands?
Hours before the the last debate in St. Louis 10 days ago, Trump appeared with women who had accused former President Bill Clinton of sexual assault, in a bid to insulate himself against a controversy over his sexually aggressive language revealed on a decade-old "Access Hollywood" video tape. His stunt led to a frosty opening with no exchange of pleasantries between the party nominees.
In any normal presidential race, the candidate who is behind -- in this case Trump -- would show up at the debate and try to mitigate the temperamental vulnerabilities that contributed to the campaign's slipping poll numbers and show a more presidential demeanor.
But Trump is no normal candidate, so there is huge uncertainty about how he will approach the final debate after scientific polls show that he lost both previous showdowns with Clinton.
He faces a clear choice heading into the clash, which will make broadcasting history as moderator Chris Wallace will be the first Fox News anchor ever to preside over an official presidential debate.
Trump could either return to the combative style he demonstrated during the first two debates, effectively taking his unrestrained performance on the campaign trail into the debate hall. Or he could seek to present a more sober personality to the American people, offering more specifics on how he would govern and displaying a more even temperament as he seeks to turn the page on recent controversies.
Since the second presidential debate, Trump has faced a string of accusations of sexual assault, all of which he has branded as lies. He also made an accusation that no candidate in modern times has dared to level -- that the election is rigged against him.
The GOP nominee has repeatedly lashed out at House Speaker Paul Ryan, who he believes is not giving him the support he deserves. As a result, Trump has declared himself "unshackled" and intends to close out his campaign free from an obligation to toe the Republican line.
He's said that Clinton should be forced to take a drug test before their final debate clash, and doubled down on his vow to put her in jail if he is elected.
Aaron Kall, editor of a new book that analyzes Trump's primary debate performances, points out that his aggressive showing in the last debate in St. Louis -- one of the most acrimonious moments in recent US political history -- came when he was still somewhat restrained.
"Now he is unshackled, what does he have to lose?" Kall said, adding that he expected Trump to be on offense most of the night.
The GOP nominee will likely skewer Clinton on his claims that hacked emails from her campaign published by Wikileaks expose wrongdoing and pay-to-play while she was secretary of state, hammer her over the private email server that has haunted her campaign, and try to position himself as the only candidate who can overthrow a reeking Washington establishment, Kall said.
He is also likely to expand on his accusations that newly released State Department documents show a senior official offered a quid-pro-quo in a fight with the FBI over the classification level of an email from her private server in a bid to shield Clinton. The Department has denied the claim.
But Trump can't afford to replicate his performances in the first two debates, when he was diverted from his most potent attacks on trade and the economy, by traps laid by Clinton or her jabs at his personality and business record.
Trump, down nine points in the latest CNN Poll of Polls, is almost out of time to launch what would be one of the most remarkable comebacks of modern times.
"Nothing he does in 90 minutes is going to cause a dramatic impact and all of the polls are going to be tied again," said Kall, Director of Debate at the University of Michigan. "But the first step is to have a good debate -- and the media loves a comeback story."
Todd Graham, debate director at Southern Illinois University, said Trump also needs to avoid behavior and wild statements in the debates that end up being endlessly repeated and fact checked afterward and lead to damning assessments of his performance.
"What ails him is he has got too much hyperbole and he has got too many falsehoods in debates," Graham told CNN's Brooke Baldwin on Tuesday.
Like Trump, Clinton has choices to make.
Does the Democratic nominee seek to protect her lead with only three weeks to go in the race or will she slug it out with Trump in a nasty, personal confrontation, as she did during the first two debates?
Clinton could try to hover above the fray, portraying herself as a president-elect in waiting, and seek to make the kind of emotional and philosophical connection she has yet to really forge with the American people through decades on the political stage and a grueling campaign that has dragged on for over a year-and-a-half.
Such an approach might enable Clinton to begin to build political momentum for her transition and early months of her presidency if she wins on November 8, but could also open her to charges of complacency before she has closed Trump out.
Unlike Trump, who has been touring the country and holding packed rallies, Clinton has been off the campaign trail for days, characteristically preparing for the debate.
The Republican nominee could not resist one of his now characteristic swipes at the former secretary of state's habit of huddling with aides to carry out mock debates.
"Tomorrow night's going to be interesting. Now she's home sleeping, and I am working. So that's the way it's going to be in the White House too. She'd be sleeping, I'd be working," Trump said at a rally in Colorado on Tuesday night.
Trump did, however, hold some debate prep sessions this time. Republican National Committee chair Reince Priebus asked questions as he portrayed a moderator while New Jersey Gov. Chris Christie stood in as Clinton.
Clinton rebuked Trump during the first debate by saying that she not only prepared for debates, she had also prepared to be President. And her communications director, Jennifer Palmieri, said Tuesday that the Democratic nominee would be ready whichever strategy Trump adopts at the final debate.
"If he chooses to continue to embrace his strategy of a scorched earth campaign ... she'll be prepared to handle that as she has the last two times," Palmieri said. "What we have seen is when he does do that, the character of Hillary Clinton that's revealed to voters is someone that is quite capable of standing up to him and defending American values."
Trump's claims that the election is "rigged" have offered Clinton a new opening to question his character and paint him as a radical departure from any President in modern history.
Clinton may also seize on his use of campaign emails stolen by Wikileaks to claim that he is manipulating private information allegedly procured by Russian intelligence services to influence the US election. She could point to that to further paint Trump as overly friendly with a US foe, Russian President Vladimir Putin.
Such charges play into Clinton's wider campaign narrative about Trump: that he lacks the temperament and in depth policy knowledge required of a commander in chief -- a characterization that the GOP nominee has to some extent helped validate with his volatile debate performances.
As in previous debates, Clinton and Trump are playing mind games in advance of their showdown. The GOP nominee invited Patricia Smith, the mother of Benghazi victim Sean Smith, who has accused the former secretary of state of "murdering" her son.
Clinton is bringing Mark Cuban, the billionaire who has feuded with Trump, to sit in the audience. But it's hard to see two such seasoned performers being knocked off their game by such transparent tactics.
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