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Donald J. Trump said on Tuesday night that he no longer vowed to support the Republican nominee if it isn’t him, despite a loyalty pledge that all Republican primary candidates signed last year.

“No, I don’t anymore,” Mr. Trump said at a town hall forum on CNN when prompted by the moderator, Anderson Cooper. “No, we’ll see who it is.”

When Mr. Cooper pointed out that Senator Ted Cruz of Texas, Mr. Trump’s chief rival for the nomination, had walked up to the line but not crossed it in terms of saying he wouldn’t support the nominee, Mr. Trump replied, “He doesn’t have to support me.”

The senator — whose wife Mr. Trump threatened to “spill the beans” about after a “super PAC” formed to stop his candidacy ran an ad featuring an old photograph of his wife, Melania, a former model — stopped short of saying he wouldn’t support Mr. Trump. Instead, Mr. Cruz said that such a situation would not come to pass because he will be the nominee.

Gov. John Kasich of Ohio was more explicit, saying that if the nominee is someone who “is really hurting the country and dividing the country,” then he just wasn’t sure. Pressed by Mr. Cooper as to whether he was saying he thinks that is what Mr. Trump is doing, Mr. Kasich declined to elaborate.

Last September, the Republican National Committee chairman, Reince Priebus, asked Mr. Trump to sign a loyalty oath at a time when he left open the possibility of bolting from the party and running as a third-party candidate. Mr. Trump said he would sign, so long as all of the other candidates did the same. So they all did.

But Mr. Trump, amid intense efforts to derail his march toward the nomination in a race in which he has a large lead among delegates to the Republican National Convention, said at the forum that he did not believe he was being treated fairly.

“I have tremendous support right now from the people,” Mr. Trump said. “I have many more delegates than him,” he added of Mr. Cruz.

“I don’t want people to do something against their will, Anderson,” he said.

Mr. Trump also gave a lengthy defense of his campaign manager, Corey Lewandowski, who was arrested on misdemeanor battery charges over an episode in which he is accused of grabbing and bruising a reporter speaking with Mr. Trump after a news conference.

“She’s not a baby,” Mr. Trump said of the reporter, suggesting that she had overstated what happened.

When Mr. Cooper asked if Mr. Trump had ever apologized for anything, he replied: “Oh, wow. I don’t know. I’ll think. Can I think?”

He added, “I do believe in apologizing if you’re wrong.” And then conceded, “I apologize to my wife for not being presidential on occasion.”