The 70-year-old Manhattan real estate mogul is behind his Democratic rival in national polls, behind in key swing states and is battling for parity in states like Arizona that should be solidly Republican.
He has been exposed making sexually predatory remarks, accused of groping multiple women, of not paying any income taxes and of being in league with Russian President Vladimir Putin.
In what has been a toxic campaign, the two White House hopefuls got off to a subdued and oddly substantive start to the debate, moderated by Fox News journalist Chris Wallace.
They were asked about their vision for the Supreme Court, prompting Clinton to argue the election was about "what kind of country are we going to be."
She insisted gay rights and women's rights must not be rolled back.
Trump echoed conservatives who believe "the Supreme Court is what it's all about," vowing to appoint anti-abortion justices who would also protect gun rights.
"If you go with what Hillary is saying, you can take the baby and rip the baby out of the womb of the mother just prior to the birth of the baby," he said.
"Using that kind of scare rhetoric is just terribly unfortunate," Clinton responded.
"You should meet with the women I've met with," she said. "This is one of the worst possible choices that any woman and her family has to make."
- 'New Brexit' -
Pundits have declared the presidential race all but over.
But Trump has shown a willingness to climb over bodies to emerge on top.
The provocative billionaire has attacked leaders of his own party and obliterated the normal rules of political decorum.
Nothing, it seems, is out of bounds. He has prompted anger and concern by questioning the legitimacy of the election.
In response, President Barack Obama implored him to "stop whining" and Senator Elizabeth Warren, a darling of the Democratic left, told him to "put on your big boy pants."
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.
Trump predicts an electoral surprise, or "new Brexit," when Americans vote next month.
But it remains an open question whether any of these stunts will have a positive impact with voters.
Clinton leads by more than six points in an average of national polls compiled by RealClearPolitics.
Women especially have thrown their support to the 68-year-old former secretary of state, who is poised to become the first female president in American history.
A Quinnipiac University poll showed she is winning women by 52 percent to Trump's 38 percent
- Mud-slinging -
Team Trump set the tone ahead of the political battle, with campaign CEO and right-wing agitator Steve Bannon promising an invitation to Obama's estranged half-brother Malik was "just an appetizer."
The pro-Trump Breitbart website also published allegations by a former television reporter that Bill Clinton sexually assaulted her three times while serving as governor of Arkansas.
Clinton's spokesman Brian Fallon said he "wouldn't be surprised" if Trump raised those claims during the debate, however cautioning that Trump's scorched earth strategy could backfire.
"If he brings this approach to the debate stage tonight, it will be his loss," he said.
In the past two debates, the duel quickly descended into highly personal attacks, pushing substantive policy issues to the side.
Clinton, a former first lady and senator, has kept a low profile in recent days, shutting herself in with aides to prepare for the onslaught.
She "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.
Trump undoubtedly will draw on the lessons of the past two head-to-head battles, and sharpen his attacks on Clinton over Syria and Libya.
The 2012 jihadist attack on the US mission in the Libyan city of Benghazi remains a burning issue among the Republican base.
Many hold her responsible for the deaths of four Americans, including ambassador Chris Stevens.
Trump has invited Patricia Smith, the mother of an American killed in the attack, to attend the debate, campaign manager Kellyanne Conway said.
But Conway advised Trump to "focus" on the issues, and come out strong against Clinton herself.
Trump also is likely to hammer Clinton over closed-door speeches to Wall Street, disclosed in hacked emails.
-----------------------------------------------------------------------------------------------------------------------
He has been exposed making sexually predatory remarks, accused of groping multiple women, of not paying any income taxes and of being in league with Russian President Vladimir Putin.
In what has been a toxic campaign, the two White House hopefuls got off to a subdued and oddly substantive start to the debate, moderated by Fox News journalist Chris Wallace.
They were asked about their vision for the Supreme Court, prompting Clinton to argue the election was about "what kind of country are we going to be."
She insisted gay rights and women's rights must not be rolled back.
Trump echoed conservatives who believe "the Supreme Court is what it's all about," vowing to appoint anti-abortion justices who would also protect gun rights.
"If you go with what Hillary is saying, you can take the baby and rip the baby out of the womb of the mother just prior to the birth of the baby," he said.
"Using that kind of scare rhetoric is just terribly unfortunate," Clinton responded.
"You should meet with the women I've met with," she said. "This is one of the worst possible choices that any woman and her family has to make."
- 'New Brexit' -
Pundits have declared the presidential race all but over.
But Trump has shown a willingness to climb over bodies to emerge on top.
The provocative billionaire has attacked leaders of his own party and obliterated the normal rules of political decorum.
Nothing, it seems, is out of bounds. He has prompted anger and concern by questioning the legitimacy of the election.
In response, President Barack Obama implored him to "stop whining" and Senator Elizabeth Warren, a darling of the Democratic left, told him to "put on your big boy pants."
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.
Trump predicts an electoral surprise, or "new Brexit," when Americans vote next month.
But it remains an open question whether any of these stunts will have a positive impact with voters.
Clinton leads by more than six points in an average of national polls compiled by RealClearPolitics.
Women especially have thrown their support to the 68-year-old former secretary of state, who is poised to become the first female president in American history.
A Quinnipiac University poll showed she is winning women by 52 percent to Trump's 38 percent
- Mud-slinging -
Team Trump set the tone ahead of the political battle, with campaign CEO and right-wing agitator Steve Bannon promising an invitation to Obama's estranged half-brother Malik was "just an appetizer."
The pro-Trump Breitbart website also published allegations by a former television reporter that Bill Clinton sexually assaulted her three times while serving as governor of Arkansas.
Clinton's spokesman Brian Fallon said he "wouldn't be surprised" if Trump raised those claims during the debate, however cautioning that Trump's scorched earth strategy could backfire.
"If he brings this approach to the debate stage tonight, it will be his loss," he said.
In the past two debates, the duel quickly descended into highly personal attacks, pushing substantive policy issues to the side.
Clinton, a former first lady and senator, has kept a low profile in recent days, shutting herself in with aides to prepare for the onslaught.
She "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.
Trump undoubtedly will draw on the lessons of the past two head-to-head battles, and sharpen his attacks on Clinton over Syria and Libya.
The 2012 jihadist attack on the US mission in the Libyan city of Benghazi remains a burning issue among the Republican base.
Many hold her responsible for the deaths of four Americans, including ambassador Chris Stevens.
Trump has invited Patricia Smith, the mother of an American killed in the attack, to attend the debate, campaign manager Kellyanne Conway said.
But Conway advised Trump to "focus" on the issues, and come out strong against Clinton herself.
Trump also is likely to hammer Clinton over closed-door speeches to Wall Street, disclosed in hacked emails.
-----------------------------------------------------------------------------------------------------------------------