DELAWARE, Ohio — Donald Trump said Thursday that he would accept the results of the presidential election “if I win,” but he doubled down on his threat to legally challenge the results if he loses.

“America is a constitutional republic with a system of laws. These laws are triggered in the case of fraud or in the event of a recount where it is needed. 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. Right?” Trump said at a Thursday afternoon rally at the Delaware County fairgrounds in this town just north of Columbus.

Trump opened his rally by making a “major announcement” and, at first, it sounded as though he might soften the stance he took during the final presidential debate on Wednesday night in Las Vegas, when he refused to say that he would accept the results of the election — a stance that even his staff members have declined to back.

“Ladies and gentlemen, I want to make a major announcement today: 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 — if I win,” Trump said, as the crowd cheered.

Trump said that he will follow all laws and traditions but that he would not give up his right to legally challenge the results if he believes that voter fraud swayed the results. He suggested that it was unfair for the debate moderator to ask him to commit to accepting the results of the election.

“That was sort of an unprecedented question,” Trump said. “If Al Gore or George Bush had agreed three weeks before the election to concede the results and waive their right to a legal challenge or a recount, then there would be no Supreme Court case and no Gore v. Bush,” Trump said. He added, “In effect, I'm being asked to waive centuries of legal precedent designed to protect the voters.”

As Trump has fallen in the polls, he has said that the electoral system is rigged against him and that rampant voter fraud could rob him of votes, even though documented cases of such fraud are rare. Trump said Thursday that undocumented immigrants are illegally voting in elections, even though only U.S. citizens are allowed to register to vote, and that Democrats are voting on behalf of people who have died, even though most jurisdictions regularly update their voter rolls.

“This is having nothing to do with me but having to do with the future of our country,” Trump said. “We have to have fairness.”

Trump accused Hillary Clinton's campaign of doing whatever it takes to get her elected — measures that he said could involve voter fraud. He mentioned reports that Clinton received some of the questions ahead of a debate during the primaries and said that she should resign from the race for cheating. He also referenced reports that Democratic operatives with no direct connection to the Clinton campaign hired people to violently disrupt Trump's rallies.

“This criminal behavior that violates centuries of tradition of peaceful democratic elections. A campaign like Clinton's that will incite violence is truly a campaign that will do anything to win,” Trump said, going on to call Clinton “a candidate who is truly capable of anything, including voter fraud.”

In accusing Clinton of cheating at previous debates, Trump pointed to an email leaked by WikiLeaks from the spring in which interim Democratic National Committee Chair Donna Brazile seemed to suggest that she had insight into a question that would come up at a primary forum against Sen. Bernie Sanders (Vt.), Clinton's Democratic primary rival. Brazile has denied that CNN provided her any questions in advance. She has refused to verify the authenticity of the email.

Trump said: “But I ask you, why shouldn't Hillary Clinton resign from the race? She was given these questions, studied these questions, got the perfect answer for these questions, and never said that she did something that was totally wrong and inappropriate.”

He said Thursday that Brazile should resign her post.

“It was just learned that Hillary Clinton was given the exact questions to a previous debate, word for word, by Donna Brazile, who is now under tremendous pressure to resign from the DNC, as she should be,” he said. “She should resign. How could a woman do that? That is cheating at the highest level.”

Trump said that he doesn't expect to have to challenge the results of the election because he expects to win, even though he is trailing far behind Clinton.

“Bottom line,” he said, “we're going to win. We're going to win. We're going to win so big. We're going to win so big.”

Trump's comments about voter fraud and his promise to challenge the results if he feels as though he lost unfairly were met with roaring cheers from a crowd of about 1,000. Several people said before the rally that they don't trust the U.S. election system.

“I think he's absolutely right,” said Kris Madigan, 60, a Trump supporter who lives in Westerville, Ohio. “I think that the media and the Clintons and Obama have all rigged the system and they're trying to make us all believe that she's the winner.”

Madigan continued: “I believe there's illegal immigrants voting, dead people, people voting two and three times. Maybe throwing our votes away. I don't know. I don't trust the system at all.”

Bob Schwarz, 74, agreed and said that “mainstream Republicans” have let voters down by minimizing the threat of voter fraud. He said he's confident that Republican leaders in Ohio have cleaned up voter rolls in the state, but he suspects that's not happening in all states.

“Here in Ohio, I think it's going to be clean, but there are other places in the country that, you know, I'm just concerned about,” Schwarz said, naming New York City and Philadelphia as examples.

Chris Wilcox, 24, said that it is “really intelligent” for Trump to raise concerns about voter fraud and to threaten to challenge any questionable results.

“I think that it's very American to say: 'Look, if there's evidence of voter fraud, then we're going to take a look at it,' " said Wilcox, who lives in Richwood, Ohio, and is the assistant manager at a sporting goods store. “I think very much he will accept the results if there isn't any evidence of voter fraud. So I think it's kind of blown out of proportion.”

Jose A. DelReal contributed to this report.