DELAWARE, Ohio — Mocking his critics, Donald Trump pledged Thursday to fully accept the outcome of next month's presidential election — if he wins. The Republican said he reserved the right to contest questionable results, deepening his unsubstantiated assertions that the race against Hillary Clinton could be rigged against him.

Trump's comments came a day after his stunning refusal in the final presidential debate to say whether he would concede to Clinton if he loses.

Prominent Republicans in tough re-election battles spent Thursday blasting Trump for refusing to promise that he would respect the results of the presidential election if he loses. Arizona Sen. John McCain, the 2008 GOP nominee, called the peaceful transfer of power “the pride of our country.”

“I didn't like the outcome of the 2008 election. But I had a duty to concede, and I did so without reluctance,” McCain said in a lengthy statement. “A concession isn't just an exercise in graciousness. It is an act of respect for the will of the American people.”

McCain joined other Republican senators facing tough re-election bids in condemning the remarks, including Sens. Kelly Ayotte of New Hampshire, Rob Portman of Ohio and Ron Johnson of Wisconsin.

With the presidential race slipping away from him, Trump has repeatedly raised the specter of a rigged election, despite no evidence of widespread voter fraud heading toward Election Day or in previous presidential contests. His top advisers and running mate Indiana Gov. Mike Pence have tried to soften his comments, only to watch helplessly as he plunges ahead.

Asked in Wednesday's debate if he would accept the election results and concede to Clinton if he loses, Trump said: “I will tell you at the time. I'll keep you in suspense.”

Clinton slammed Trump's comments as “horrifying,” and fellow Democrats piled on Thursday.

“That undermines our democracy,” President Barack Obama said while campaigning for Clinton in Florida. “Our democracy depends on people knowing their vote matters.”

Clinton and Trump appeared together Thursday night at the Alfred E. Smith Memorial Foundation dinner, a white-tie gala benefiting Catholic charities. The candidates traditionally tell light-hearted jokes about one another, but Trump quickly tore into Clinton.

“Last night, I called Hillary a nasty woman, but this stuff is all relative after listening to Hillary rattle on and on and on, I don't think so badly of Rosie O'Donnell anymore,” he said.

At another point, he noted: “Here she is tonight, in public, pretending not to hate Catholics,” drawing boos from the audience.

Clinton, in a lighter address, got big laughs when she told the audience: “I just want to put you all in a basket of adorables.”

Clinton added after Trump's speech that she'll “enjoy listening to Mike Pence deny that you ever gave it.”

Earlier Thursday while campaigning in must-win Ohio, Trump tried to make light of his position on concession.

“I would like to promise and pledge to all of my voters and supporters and to all of the people of the United States that I will totally accept the results of this great and historic presidential election,” he said. After letting that vow hang in the air for a few seconds, he added: “If I win.”

The Republican nominee said he would accept “a clear election result” but reserved his right to “contest or file a legal challenge” if he loses.

Trump's advisers and surrogates struggled to explain the candidate's position. Campaign manager Kellyanne Conway said it was too early to determine whether voting irregularities could make the difference between winning and losing. She and other Trump backers drew a parallel to then-Vice President Al Gore's concession call to then-Texas Gov. George W. Bush, which he later withdrew as he awaited a recount in Florida.

Reince Priebus, chairman of the Republican National Committee, contended that Trump and the party would stand by the results unless the margin is small enough to warrant a recount or legal challenges. Priebus said Trump is merely preserving flexibility in the event of a contested result.

The Washington Post contributed.