“I will totally accept the results of this great and historic presidential election if I win,” the Republican nominee told cheering supporters in Delaware, Ohio.
“Of course I will accept a clear election result, but I will also reserve my right to contest and file a legal challenge in the case of a questionable result.”
“I will follow and abide by all of the rules and traditions of all of the many candidates who have come before me,” he added.
The final 2016 presidential debate on Wednesday was dominated by Trump’s refusal to say he would recognise a victory by Democrat Hillary Clinton, 68, who he accuses of conspiring to rig the vote against him.
That stance is unprecedented in modern US political history and has elevated fears of post-election unrest.
Trump had one last chance at a nationally televised debate to reach out to the undecided voters he badly needs to keep his presidential campaign viable. He passed on the opportunity.
Instead, he chose to stay with the strategy he has employed during recent weeks: Pump up his hard-core supporters and hope that’s enough to win.
He suggested he might not accept the election result if Clinton wins on Nov. 8, called her a “nasty woman,” and repeated hard-line conservative positions on issues such as abortion and immigration.
While that kind of rhetoric was catnip to his passionate, anti-establishment base, it is unlikely to have appealed to independent voters and women who have yet to choose a candidate.
“When you’re trailing in the polls, you don’t need a headline the next morning saying that you’re not going to accept the election results,” said Ford O’Connell, a Republican strategist who supports Trump.
With less than three weeks left in the race, Trump is behind Clinton in most battleground states and is underperforming in almost every demographic voter group compared to the Republican nominee, Mitt Romney, four years ago.
Party strategists had said before the debate that he needed to use the event to draw in voters beyond his hard-core supporters.
Trump didn’t listen or perhaps didn’t care.
His debate was a continuation of his apparent strategy to ensure his most fervent supporters show up on Election Day, while betting that his attacks on Clinton’s character and truthfulness will discourage voting by already sceptical young and liberal Democrats.
But experts who study voter behaviour warned that his attacks on Clinton may backfire, saying he may instead awaken Democratic voters who have so far been uninspired by Clinton.
“The risk he faces by engaging in a scorched-earth policy is that he activates people rather than turning them off,” said Michael McDonald, who runs the US Election Project at the University of Florida.
It is against this backdrop that Trump has apparently decided to double down on energising his base rather than broadening it.