By Steve Holland and Amanda Becker
LAS VEGAS (Reuters) - Republican presidential nominee Donald Trump and Democrat Hillary Clinton go head to head on Wednesday night in a third and final debate as Trump tries to reverse momentum in an election that polls show is tilting away from him.
Trump has been damaged recently by several accusations of groping women - which he denies - and concerns about his claims that the election will be rigged against him.
The debate at the University of Las Vegas Nevada gives the New York businessman perhaps his best remaining chance to sway the dwindling number of Americans who are still undecided about which way to vote in the Nov. 8 election.
Tens of millions of voters are expected to tune in.
"This one is important if Trump is going to have any chance to get back into this race," said Republican strategist Charlie Black. "He's going to have to talk about issues effectively and not get down in the mud, and he needs to talk about jobs."
Trump campaign manager Kellyanne Conway acknowledged that her candidate goes into the debate, which starts at 9 p.m. EDT (0100 GMT Thursday), as the underdog.
"It's a comeback and he's done it before several times in this campaign," Conway said on Fox News, adding a jibe at Clinton: "Hillary Clinton just has not been able to put him away. What is her problem already?"
For Clinton, who leads national polls and in most of the battleground states where the election will likely be decided, the 90-minute debate is a chance to make a closing argument on why she is best suited to succeed President Barack Obama.
"I can't imagine what Donald Trump could do positively or a mistake Hillary Clinton could make to change the trajectory of this race," Democratic strategist Steve Elmendorf said.
Both candidates are likely to attack each other's character, as they have during a long and often nasty campaign.
Trump's White House bid has been marked by personal squabbles and controversial comments about immigration and minorities. Most recently, he has urged supporters to patrol polling places in inner cities to prevent voter fraud, and tried to fight off several allegations that he groped women.
Clinton has struggled to get past concerns about transparency raised over her handling of emails while she was secretary of state from 2009 to 2013.
"She needs to be able to answer the email question," Democratic strategist Bud Jackson said. "She hasn't quite hit that nail on the head yet. She should do better this time. And she should expect the unexpected."
But Trump's troubles have loomed the largest after a video from 2005 was released showing him bragging about groping women. That has weighed down the real estate magnate's political run going into the final weeks.
This has prompted him to lash out at what he calls a political and media system rigged against him, an apparent bid to discredit the process before the election takes place. Obama said Tuesday that Trump needs to "stop whining" and make his case to voters.
Trump's daughter Ivanka, who has advised the candidate, said she thought her father would "do the right thing" and accept the Nov. 8 result, no matter what it is. When asked if she thought the election was rigged, she said at a Fortune "Most Powerful Women Summit" that the media had been "vicious" about Trump.
Republicans who have watched Trump repeatedly shoot himself in the foot said the debate was a chance to appear presidential.
"Trump needs to make a real closing argument and stop the personal attacks and come across like a commander-in-chief," Republican strategist Scott Reed said.
The debate, moderated by Fox News' Chris Wallace, is to include discussions of debt and entitlements, immigration, the economy, the Supreme Court, foreign hot spots and fitness to be president.
Both candidates have invited guests who could be seen as provocative.
Trump asked Obama's estranged half-brother Malik, who supports the Republican, as well as Pat Smith, the mother of one of the four Americans killed in the 2012 attacks in Benghazi, Libya, who blames Clinton for her son's death.
Clinton's guests include Mark Cuban, the billionaire owner of the Dallas Mavericks basketball team and a frequent Trump antagonist.
Cuban wrote on Twitter that he could not wait to give "a big hug to my bestie" Trump in Las Vegas. "I know you miss me!" he added.
(Additional reporting by Emily Stephenson in Las Vegas, Patricia Zengerle in Washington and Lisa Richwine in Laguna Niguel, Calif., Editing by Leslie Adler and Alistair Bell)