Las Vegas (CNN) Donald Trump used the third and final presidential debate to dig himself into a deeper hole that only increases the threat to his presidential prospects.

Over the course of 90 minutes, Trump scored some early hits against Democratic opponent Hillary Clinton. But it's one moment that will haunt the Republican nominee and define the narrative heading into the campaign's last days.

"I will look at it at the time," Trump said when asked whether he would concede if he loses on November 8. " I will keep you in suspense ."

To emphasize the extraordinary nature of Trump's statement, Clinton responded: "That's horrifying."

The comments -- combined with a knock on Clinton as a "nasty woman" and a quip about "bad hombres" -- is certain to dominate the political conversation in the remaining 19 days of the race. Though Clinton enjoys a commanding lead in the polls, the overwhelming narrative is likely to be about Trump's potential refusal to accept defeat and whether that could trigger a historic confrontation in the days following the election.

The Republican nominee doubled down on his comments about the election Thursday during a rally in Delaware, Ohio, where he said he would accept the results "if I win."

Exasperated Republicans have no choice but to once again weather the fallout. It's an especially tough task for Senate Republicans facing voters next month, as those who have not already disowned their party's nominee could come under renewed pressure to do so.

Trump's troubles

Trump's troubles have been brewing for weeks. He has been unable to move past damaging stories about his treatment of women, including a flood of recent sexual assault accusations -- allegations he denies. Trump is now trailing Clinton in the polls and confronting a rapidly narrowing path to the White House, as even reliably red states such as Arizona have become battlegrounds.

Photos: The final presidential debate Photos: The final presidential debate Democratic nominee Hillary Clinton walks off stage following the presidential debate in Las Vegas on Wednesday, October 19. Hide Caption 1 of 22 Photos: The final presidential debate Republican nominee Donald Trump walks off stage with his family after the debate. Hide Caption 2 of 22 Photos: The final presidential debate Clinton gestures to the crowd immediately after the debate. There was no handshake between her and Trump. Hide Caption 3 of 22 Photos: The final presidential debate Trump speaks during the debate -- the third of three presidential debates this year. It took place 20 days before Election Day. Hide Caption 4 of 22 Photos: The final presidential debate Trump entered the debate in his weakest position yet in national polls. Recent national polls show Clinton's lead in the high single digits. And it doesn't look much better for Trump in several key battleground states. Hide Caption 5 of 22 Photos: The final presidential debate According to MJ Lee, CNN national politics reporter, Clinton's major challenge entering the debate was not so different from the challenge she's confronted over the past few months: presenting the country with a positive vision for her presidency that is detached from her argument against Trump. Hide Caption 6 of 22 Photos: The final presidential debate Clinton takes notes during the debate. Hide Caption 7 of 22 Photos: The final presidential debate "Fox News Sunday" host Chris Wallace was the moderator for the debate. Hide Caption 8 of 22 Photos: The final presidential debate Trump speaks to Clinton during the debate. Hide Caption 9 of 22 Photos: The final presidential debate Clinton is seen on a television screen at the debate venue. Hide Caption 10 of 22 Photos: The final presidential debate Trump listens to a Clinton answer. Hide Caption 11 of 22 Photos: The final presidential debate Trump takes notes. Hide Caption 12 of 22 Photos: The final presidential debate During the debate, Trump refused to say he would accept the result of next month's presidential election. "I will look at it at the time," Trump said when challenged on his claims that the election is "rigged" against him. Hide Caption 13 of 22 Photos: The final presidential debate Clinton arrives for the start of the debate. Hide Caption 14 of 22 Photos: The final presidential debate Clinton waves to the crowd before the debate. Hide Caption 15 of 22 Photos: The final presidential debate Wallace speaks to the debate attendees. Hide Caption 16 of 22 Photos: The final presidential debate Clinton's husband, former U.S. President Bill Clinton, attends the debate with their daughter, Chelsea. Hide Caption 17 of 22 Photos: The final presidential debate Trump's sons Eric and Donald Jr. wait for the debate to begin. Hide Caption 18 of 22 Photos: The final presidential debate Malik Obama, President Barack Obama's Kenyan-born half-brother, was one of Trump's guests. In July, Malik Obama voiced his support for the Republican. Hide Caption 19 of 22 Photos: The final presidential debate Bill Clinton waits for the third debate to start. Hide Caption 20 of 22 Photos: The final presidential debate Trump's daughter Ivanka attends the debate. Hide Caption 21 of 22 Photos: The final presidential debate Trump's wife, Melania, arrives for the event. Hide Caption 22 of 22

The swirling controversies have only emboldened Trump, who has taken to furiously lashing out in all directions: at his accusers, the media, GOP leadership and, most recently, the political system. He is now outlandishly warning -- with no evidence -- that the election is "rigged" in favor of Clinton.

When questioned about the women who have come forward to accuse him of having inappropriately touched them, a visibly irritated Trump was defiant, going as far as to say he believes the women are part of a Clinton set-up.

"Those stories have been largely debunked," Trump said, despite the fact that some of the women's stories are corroborated by others. "It was lies and it was fiction."

Clinton seized the moment to remind viewers that Trump has insulted the physical appearance of his accusers.

"He said that he could not possibly have done those things to those women because they were not attractive enough to be sexually assaulted," she said.

These exchanges came after what had at first been a relatively subdued and policy-heavy debate. But it did not take long -- or much -- for Trump to lose his composure as he repeatedly interrupted Clinton and rolled his eyes from behind the podium.

At one point, Trump even lashed out by calling Clinton "such a nasty woman." The comments were particularly remarkable given that they were discussing what should have been a relatively sleepy topic: entitlements.

Missed opportunity

Ultimately, the debate was one more missed opportunity for Trump to tackle his biggest handicap -- the widespread perception that he lacks the temperament to be president. That could very well prove to be fatal. Trump won't have another opportunity before November 8 to reach the sheer number of people who have tuned into the debates.

Even before Trump's disastrous debate performance Wednesday night, there was growing evidence that the past few tumultuous weeks had taken a serious toll on his campaign.

Clinton now leads her GOP rival by eight points among likely voters, according to CNN's Poll of Polls released Wednesday. Trump's public image has also taken a hit from the recent scandals: A Bloomberg survey released on the day of the debate showed that Trump's unfavorability rating had gone up to 62% from 56% a month ago (Clinton's ticked down to 52% from 56% in September).

And Trump's path to 270 electoral votes is also looking increasingly tough. Key battleground states including Florida and Nevada are now leaning in Clinton's favor, while traditionally Republican strongholds like Arizona are now in play for Clinton.

While Trump has tried to stay afloat, Clinton has largely stayed the course. She has let Trump stew in his controversies while continuing to hammer the message that he is fundamentally unfit to be president. Her campaign has also recalibrated its resources to seize on Trump's falling momentum by expanding their map and focusing on early voting and get-out-the-vote efforts.

Trump's standing in the 2016 race has grown progressively worse through the three debates.

In his first match-up against Clinton , Trump was consistently on the defensive, as his challenger needled him on everything from his temperament, refusal to release his tax returns and past comments about race and women.

It was clear that Trump's lack of intense debate preparations -- including his refusal to hold mock debates -- was damaging, particularly as he went up against a veteran and practiced candidate like Clinton. Next to his opponent, Trump appeared angry and erratic.

Heading into the second debate in St. Louis last weekend, Trump's critical mission was to slow the bleeding.

Last straw

The damning "Access Hollywood" tape had become the last straw for many of Trump's fellow Republicans, who finally cut ties with the controversial nominee. Things only grew worse when multiple women came out to accuse Trump of exactly the kind of behavior he was heard describing on that tape.

But rather than using the second debate to express genuine contrition, Trump tripled down. He convened a surprise media gathering featuring women who have accused Bill Clinton of sexual misconduct, before saying on the debate stage: "Bill Clinton was abusive to women. Hillary Clinton attacked those same women and attacked them viciously."

Following the debate, even Trump's top adviser struggled to answer for her candidate's refusal to say he would accept the results of the election.

"Donald Trump will accept the results of the election because he's going to win the election," campaign manager Kellyanne Conway told CNN's Dana Bash.

Pressed by Bash on why Trump didn't say that on the debate stage -- and whether Conway will personally urge Trump to accept the outcome -- Conway grew impatient.

"Absent evidence of widespread abuse and irregularities, yes, I would say that, but I actually think I'll be saying to him: 'Congratulations, Mr. President,'" she said.