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With just 20 days before Americans cast ballots November 8, polls showed Clinton widening her lead over the provocative billionaire.
The 90-minute debate at the University of Las Vegas, Nevada offers Trump what may be his last chance to roll the political dice and reverse a campaign that has been battered by a stream of allegations he groped and forced himself on women.
"This is our final shot, folks," Trump told supporters Tuesday in Colorado.
WATCH: SBS Senior Correspondent Brett Mason live from the third Presidential Debate in Las Vegas with POLITICO Correspondent Hadas Gold
Clinton has kept a low profile in recent days, shutting herself in with aides to prepare for the third televised debate of the campaign.
Moderated by Fox News journalist Chris Wallace, the event begins at 0100 GMT, before a televised audience expected to top 50 million.
The candidates will be asked about the economy, immigration, the Supreme Court, foreign flashpoints and their fitness to be president.
The past two debates quickly degenerated into highly personal attacks that pushed substantive policy issues to the side.
Clinton "should do what she did in the two first debates which was largely remain calm, deflect criticism and attacks and let Donald Trump continue to self destruct," said John Hudak, a senior fellow at the Brookings Institution.
"If your opponent's ship is sinking you don't throw them a life preserver," he said.
Source: AFP
Rattle Clinton?
Trump undoubtedly will draw on the lessons of the past two debates, and sharpen his attacks on Clinton over Syria and Libya.
The 2012 jihadist attack on the US mission in Benghazi, Libya remains a burning issue among Republicans, who hold the former secretary of state partially responsible for the deaths of four Americans, including the ambassador.
Trump has invited Patricia Smith, the mother of an American killed in the attack, to attend the debate, as well as President Barack Obama's Kenyan-born half-brother Malik, who supports Trump, campaign manager Kellyanne Conway said.
The move is a clear ploy to rattle the former secretary of state, much like Trump sought to do in the previous debate when he invited women who had accused former president Bill Clinton of past sexual misconduct.
New allegations emerged Wednesday, from a former reporter who claims Clinton groped her in 1980 when he was governor of Arkansas -- potentially providing new fodder for Trump to attack the Democratic nominee over her husband's past.
But Conway advised Trump to "focus" on the issues, and come out strong against Clinton herself.
She is "a known liar," Conway told MSNBC. "She's lied to Congress. She's lied to the FBI. She lied to the Benghazi families. She lies and it's always for political gain."
Trump is likely to hammer Clinton over recent revelations from emails hacked from her campaign staff and released by WikiLeaks that exposed her closed-door speeches to Wall Street and showed her deliberations over free trade.
But Republican Senator Marco Rubio, acknowledging US government concerns that Russia had orchestrated the hacks, issued a stern warning about such a tactic.
"These leaks are an effort by a foreign government to interfere with our electoral process and I will not indulge it," Rubio said in a statement.
"Today it is the Democrats. Tomorrow it could be us."
'Stop whining'
An average of recent national polls shows Clinton with a seven point lead over Trump when the race includes third-party candidates Gary Johnson and Jill Stein.
Trump predicts an electoral surprise, or "new Brexit," when Americans vote next month.
But as he's lost ground, the New York real estate mogul has attacked the election process itself, complaining of a conspiracy to steal the election from him.
"The press has created a rigged system and poisoned the minds of the voters," he said Tuesday.
"Either we win this election or we lose the country."
The White House is increasingly concerned that Trump and his supporters will not recognize the election's outcome, plunging the country into a political crisis.
In a scorching rebuke, Obama told Trump on Tuesday to "stop whining" and go make his case to voters.
"I have never seen in my lifetime or in modern political history any presidential candidate trying to discredit the elections and the election process before votes have even taken place. It's unprecedented," the president said.
Ohio Governor John Kasich, who lost to Trump in the Republican primaries, weighed in to slam his talk of election rigging.
"That's like saying we never landed on the moon," Kasich told CBS News.
