LAS VEGAS - Trailing in the polls, Donald Trump had little to lose in his final debate against Hillary Clinton. He still managed to send shock waves, refusing to say he would accept the results of the 2016 election if he loses.

Trump largely steered clear of making personal attacks against Clinton at the start of the debate, but repeatedly clashed with the former secretary of state as it went on. He called her a liar, questioned her experience and at one point labeled her "such a nasty woman."

Unconventional as always, the celebrity businessman wouldn't commit to conceding the election if he fails to win the White House, a staggering declaration by a presidential nominee.

Some takeaways from Wednesday's debate:

NO SURRENDER

More than an hour into the debate, Trump would not commit to honouring the election results if Clinton wins. Pressed by moderator Chris Wallace, Trump said he would "keep you in suspense."

Clinton said it was "horrifying" that Trump wouldn't heed the nation's tradition of having a peaceful transfer of power. The businessman has argued without providing any evidence that the election is rigged against him and has suggested Clinton would be an illegitimate president because she used a private email server as secretary of state. He said she "never should have been allowed to run."

'NASTY WOMAN'

Trump faces the possibility of a large gender gap against Clinton and some of his commentary is unlikely to help him with women voters in swing states.

At the end of the night, Trump referred to Clinton as a "nasty woman" and denied again that he had engaged in any sexually aggressive behaviour, blaming Clinton's campaign for the series of women who have charged him with groping and other forms of harassment and assault in recent days. The GOP candidate said he didn't know his accusers.

"I didn't even apologize to my wife," Trump said, "because I didn't do anything." In an interview earlier this week, Melania Trump said her husband apologized to her.

Clinton didn't let the moment go, noting that Trump had derided the women as unattractive and "disgusting." Clinton said, "Donald thinks belittling women makes him bigger."

BAD HOMBRES

When Trump reiterated his call to build a wall along the Mexican border, he vowed again to get rid of "drug lords" and "bad people" in the country. Adding a touch of Spanish, Trump said, "we have some bad hombres here, and we're going to get them out."

The phrase quickly resonated on social media with allies of Clinton saying Trump was denigrating immigrants.

Clinton said Trump's immigration policies would amount to "rounding up people who are undocumented" and would "rip our country apart."

ABORTION

The candidates had their first extended exchange over abortion rights of the general election. Trump said he thought Roe vs. Wade would "automatically" be overturned if he is elected because he will appoint justices who are against abortion rights. But he refused to say whether he personally thinks the landmark case should be overturned.

Clinton said she'd strongly defend the ruling and offered a passionate defence of a woman's right to have an abortion, even as Trump accused her of supporting "ripping" babies from their mothers' wombs.

"This is one of the worst possible choices that any woman and her family has to make, and I do not believe the government should be making it," Clinton said.

PUTIN

A question over immigration quickly turned into a sharp exchange over Russia's role in the election, an issue that has become central to the campaign.

Asked about her apparent support for "open borders" in a private speech to a Brazilian bank, Clinton pointed out that the excerpt came from a hack of her campaign chairman's personal email - an intrusion that U.S. intelligence agencies blame on Russia. She called on Trump to "reject Russian espionage against Americans" and cast him as a "puppet" of Vladimir Putin.

Trump shot back that Clinton was the "puppet," saying "of course" he condemns Russia or any other country interfering in the U.S. elections. But Trump said he doubted that Russia had hacked emails from her campaign. Trump went onto say that Putin is not "my best friend." He added, "She doesn't like Putin because Putin has outsmarted her at every step of the way."

'BAD EXPERIENCE'

At the end of President Barack Obama's two terms, Trump tried to reach voters who may be in the mood for a change in Washington. He cast himself as an outsider, acknowledging to Clinton: "The one thing you have over me is experience. But it's bad experience."

Trump said, "The problem is, you talk, but you don't get anything done, Hillary."

Clinton responded by comparing her record over the past three decades to Trump's resume as a businessman. She pointedly noted that on the day she was inside the White House Situation Room during the raid that killed Osama bin Laden, Trump was hosting his NBC reality show, "Celebrity Apprentice."

TOO LATE?

Trump did not appear to do enough to connect with women, college-educated and suburban voters who have gravitated to Clinton during the fall and helped her build a solid lead in preference polls.

His unwillingness to say he'll accept the results of the election should he lose will give Clinton another way to argue that he doesn't have the temperament to be president.

And now Trump has little time to make up ground. When the debate got underway, about 2.2 million Americans had already voted and many more will take part in in-person early voting next week. Clinton's campaign expects more than half the electorate in the battleground states of Florida, North Carolina and Nevada will vote before Election Day.

For many voters, the debate may have been the final chance for Clinton and Trump to make their case.