Donald Trump and Hillary Clinton challenge battle in the final US presidential debate

Hillary Clinton and Donald Trump's ugly and acrimonious battle for the White House is barrelling toward the end, after the candidates took the debate stage today for one final prime time showdown.

Donald Trump and Hillary Clinton began the last debate without a handshake - and they ended it without one too.

On the election result

The main takeaway from today's debate seems to be Trump's refusal to say whether he will accept the election result if he loses.

"I will tell you at the time," Trump told moderator Fox News' Chris Wallace, after Wallace asked whether Trump would follow the American tradition of accepting the results of presidential elections.

Clinton interjected."I will keep you in suspense."

"Chris, let me respond to that," she said. "That's horrifying."

Trump also referenced what he said was a "rigged" election: He blamed the news media for "poisoning" minds against him, and the FBI for not prosecuting Clinton for her use of a private email server to handle government business while she was secretary of state.

On women

The topic of Trump's treatment of women also figured large.

He rejected the accounts of women who have accused him of groping them or kissing them without their consent.

"I didn't even apologise to my wife, who's sitting right here, because I didn't do anything," Trump said, when asked by Wallace about nine women who have come forward in recent days.

Clinton said that Trump's treatment of women reflected a broader flaw in his personality.

"Donald thinks belittling women makes him bigger. He goes after their dignity, their self-worth," Clinton said. "And I don't think there's a woman anywhere who doesn't know what that feels like."

On ties with Russia

And earlier in the debate, Clinton accused her opponent of being a "puppet" of Russian leader Vladimir Putin.

"It's pretty clear, he'd rather have a puppet as president of the United States," Clinton said, noting that Russian hackers had been blamed for releasing internal Democratic emails, apparently in an effort to help Trump's cause.

"You're the puppet," Trump replied.

He argued that Putin had outmanoeuvred Clinton - who was previously secretary of state - and President Barack Obama in nuclear treaties, and in Middle Eastern conflicts like the one in Syria.

"She doesn't like Putin because Putin has outsmarted her at every step of the way," Trump said.

A last opportunity for Trump

For Trump, the debate was perhaps his last opportunity to turn around a race that appears to be slipping away from him. His predatory comments about women and a flood of sexual assault accusations have deepened his unpopularity with women and limited his pathways to victory.

Clinton took the stage facing challenges of her own. While the electoral map currently leans in her favour, the Democrat is facing a new round of questions about her authenticity and trustworthiness, concerns that have trailed her throughout the campaign.

The Democratic vice-presidential nominee said running mate Clinton would be cool and collected in the face of Trump's likely insults and histrionics.

Meanwhile, Republican vice-presidential candidate Mike Pence was predicting the debate would revolve around "bigger things" facing the nation, rather than controversies surrounding Clinton.

Psychological tactics

Both presidential campaigns have been using their debate guests as a tool to try to get inside their rivals' heads.

Trump is bringing Barack Obama's half-brother, Malik Obama, a Trump supporter. Clinton is bringing frequent Trump critic and billionaire Mark Cuban and Hewlett Packard Enterprise CEO Meg Whitman.

No matter how persuasive Trump and Clinton are, it's already too late for either to win over millions of voters.

Advance voting by mail or in person is now underway in more than 30 states. At least 2.1 million voters have already cast ballots.

More than 45 million people are expected to vote before Election Day, November 8.

The third and final debate of the caustic 2016 US presidential election started with a sober tone... but ended up with Donald Trump questioning the fundamental underpinnings of the nation. Here are four key moments:
If Americans entered the debate concerned about what happens the day after this brutally fought election, Trump did little to assuage their fears.
An hour into the 90-minute battle, the Republican nominee was asked whether he would respect the election result and concede if he lost. His answer will go into the history books.
"I'll look at it at the time. What I've seen is so bad," he said, repeating unfounded allegations of vote rigging.
Fellow Republicans rushed to denounce him, and his Democratic rival Hillary Clinton declared herself "appalled" by what she said was an attack on 240 years of US democracy.
Asked about embarrassing leaked emails, Clinton pivoted to Trump's much scrutinized relationship with Russian President Vladimir Putin and the Republican's refusal to pin the leaks on Moscow.
Clinton suggested Putin wanted a "puppet" in the White House, which prompted a remarkable exchange.
"No puppet. No puppet," said Trump, talking over Clinton. "You're the puppet!"
"No, you're the puppet," he continued.
Composing himself, Trump said "I never met Putin. This is not my best friend. But if the United States got along with Russia, it wouldn't be so bad."
Both Clinton and Trump threw plenty of meat to their core political bases -- ticking boxes on guns, abortion and taxes.
Trump was again on the defensive over his policy of forcibly deporting millions of illegal migrants.
"We have some bad hombres here, and we're going to get them out," he said.
Clinton described that as an idea "that would rip our country apart."
Some of the sharpest exchanges came when Trump accused Clinton and her campaign team of drumming up allegations that he has groped several women.
"I believe," Trump said, "she got these people to step forward," accusing Clinton of running a "very sleazy campaign" and adding of the claims aired by several women dating back decades: "It was all fiction."
Trump boasted, "I didn't even apologize to my wife," saying he did nothing wrong and so had nothing to apologize for.
Later, when the topic turned to taxes, Clinton suggested that Trump might try to wriggle his way out of paying.

"Such a nasty woman," Trump said, leaning into the microphone.