During the debate, Fox News' Chris Wallace, who was moderating, asked Trump, "Do you make the same commitment that you will absolutely -- sir, that you will absolutely accept the result of this election?"
Trump responded, "I will look at it at the time. I'm not looking at anything now. I'll look at it at the time."
He continued by saying "the media is so dishonest" and that "millions of people that are registered to vote that shouldn't be registered to vote."
"And just in that respect, I say it's rigged," Trump said.
Republican South Carolina Sen. Lindsey Graham, who ran against Trump in the GOP presidential primary, slammed Trump on Twitter, writing in a statement, "During this debate Mr. Trump is doing the party a disservice by continuing to suggest the outcome of this election is out of his hands and 'rigged' against him. If he loses, it will not be because the system is 'rigged' but because he failed as a candidate."
Trump, who's been facing a decline in polls, widely panned debate performances and recent sexual allegations from women, has increasingly suggested that the election is "rigged" against him -- and he didn't back down from that during Wednesday's presidential debate.
The Republican nominee has also urged his supporters to show up at polling places to ensure no voter fraud occurs, which is an order that appears to risk voter intimidation.
Republican Arizona Sen. Jeff Flake, who has also been a strong critic of Trump, tweeted, ".@realDonaldTrump saying that he might not accept election results is beyond the pale"
And Florida Republican Rep. Carlos Cuberlo also took to Twitter to write, "Peaceful transfer of power & acceptance of election results is fundamental to our democracy & Constitution. This cannot be undermined ever."
Jeff Roe, the man who was campaign manager of Texas Sen. Ted Cruz's presidential campaign, encouraged Trump to "hold it together, man."
Additionally, conservative commentator Laura Ingraham, who has been a key Trump supporter, also weighed in, tweeting, "He should have said he would accept the results of the election. There is no other option unless we're in a recount again."