Trump's refusal, which Democratic rival Hillary Clinton called "horrifying," was the standout remark of their third and final debate on Wednesday night. It ratcheted up Trump's claims that the election was being rigged against him, and became the latest flashpoint in an unusually volatile race three weeks before voters go to the polls.
The Republican candidate reinforced his comment at a rally in Delaware, Ohio, on Thursday, saying he would respect the result "if I win."
"Of course I would accept a clear election result, but I would also reserve my right to contest or file a legal challenge in the case of a questionable result," he said.
"I will totally accept the results of this great and historic presidential election...if I WIN." - @realDonaldTrump— Emily Stephenson (@ewstephe) October 20, 2016
With Trump trailing in opinion polls, the focus ahead of the Nov. 8 vote is shifting to Congress, and whether Republicans will keep their narrow majority in the Senate or even their larger advantage in the House of Representatives.
Senator John McCain of Arizona, who lost the 2008 presidential election to Democrat Barack Obama, said accepting the election result is "the American way."
"I didn’t like the outcome of the 2008 election. But I had a duty to concede, and I did so without reluctance," McCain, who has opened a poll lead in his Senate re-election race, said in a statement. "A concession isn't just an exercise in graciousness. It is an act of respect for the will of the American people, a respect that is every American leader's first responsibility."
McCain has withdrawn his support for Trump.
Asked on Wednesday night if he would commit to a peaceful transition of power, the businessman-turned-politician replied: "What I'm saying is that I will tell you at the time. I'll keep you in suspense. OK?"
Trump's statement, the most controversial in a debate that at times descended into insults by both candidates, made banner headlines across the country and raised questions about whether he was committed to a peaceful transition of power, a cornerstone of American democracy.
Democrats jumped to ask Republican candidates whether they agreed with Trump, who is making his first run for public office against Clinton, a former first lady, senator and secretary of state.
"Do you agree with Donald Trump to question the results of the election?" the Nevada Democratic Party asked in a release targeting Republican Representative Joe Heck. Heck is in a tight race with Democrat Catherine Cortez Masto, a former Nevada attorney general, for the Senate seat held by retiring Democratic leader Harry Reid.
DROWNING OUT POLICY
Republican consultant Matt Mackowiak said down-ballot candidates would distance themselves from Trump's comment, but it was a problem that the issue had drowned out everything else that came up on Wednesday.
"The real cost is that the post-debate discussion has been consumed by this, and not by his overall very good debate performance and the problems Hillary created for herself on a range of issues," Mackowiak said.
A CNN/ORC snap poll said 52 percent thought Clinton won the debate, to 39 percent for Trump.
Trump donor and energy investor Dan Eberhart thought Trump won, but disagreed with his rhetoric, although he is sticking with the candidate.
"I think Hillary’s policies and track record are not what the country needs leading us forward for the next four years. And that backs me into supporting Trump," Eberhart said.
Millions of Americans watched the debate in Las Vegas. However, the television audience was below Clinton and Trump's record-setting first debate, according to early data cited by U.S. media outlets.
On Twitter, President Obama said Clinton had scored an "Outstanding 3 for 3 debate sweep." Obama has described Trump as unfit for the White House.
Trump's words were considered jaw-dropping, but they are not illegal, especially given the strong guarantee of speech rights in the United States.
Trump campaign manager Kellyanne Conway on Thursday tried to defend Trump, saying in television interviews he was "putting people on notice" about voting irregularities.
Trump has stepped up allegations that the election is being rigged. He has not offered specific evidence, and numerous studies have shown that the U.S. election system, which is run by the states, is sound.
His comments about vote rigging have come against a backdrop of accusations that Trump had made improper sexual advances to women. A tenth woman came forward on Thursday at a news conference in midtown Manhattan with her attorney, Gloria Allred, a Clinton supporter who for years has specialized in representing women in cases of alleged assault.
Trump campaign spokeswoman Jessica Ditto called the news conference a "coordinated, publicity-seeking attack" by Allred.