Sliding in the polls with just weeks left in the campaign, Donald Trump has ramped up claims that the election is "rigged" and could be susceptible to widespread voter fraud.But during the debate on Wednesday, the GOP nominee took those fear-mongering — and largely unsubstantiated — claims a step further, declining to say whether he'll accept the final results on November 8."What I'm saying is: I'll tell you at the time," Trump told moderator Chris Wallace during the presidential debate. "I'll keep you in suspense."Actual instances of voter fraud at the polls are incredibly rare . And some experts worry that the nominee's calls for supporters to "go out and watch" the polls could lead to voter intimidation and suppression.It would be unprecedented for a candidate for president to refuse to accept the final outcome of the election. Wallace noted that in the question that prompted Trump's response: "There is a tradition in this country, in fact one of the prides of this country, is the peaceful transition of power and that no matter how hard fought a campaign is, that at the end of the campaign, the loser concedes to the winner...and that the country comes together, in part, for the good of the country."Trump's comments contradict those of some of his closest campaign advisers, including his daughter, Ivanka, and campaign manager, Kellyanne Conway. Both women have said the campaign will accept the results of the election. Hillary Clinton summed up her reaction to her rival's latest claim in one word: "Horrifying."Watch the full exchange in the video, above.