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MARSHALLTOWN, Iowa — Moments after Fox News announced the lineup for Thursday’s Republican debate, Donald J. Trump said Tuesday that it was pretty “irrevocable” that he would skip the event, which takes place just days before the Iowa caucuses.

“Let’s see how much money Fox is going to make on the debate without me,” Mr. Trump said at a news conference here, where he continued to attack Megyn Kelly and the crew at Fox News, with whom he sparred at an earlier debate.

“It’s time that somebody plays grown-up,” he said.

Just after Mr. Trump started speaking, his campaign manager, Corey Lewandowski, confirmed that he would skip Thursday’s event, saying, “He will not be participating in the Fox News debate on Thursday” and that “it’s not under negotiation.”

Mr. Trump’s absence would leave the main debate stage without the candidate who leads in national polls, one who has been a big ratings draw for past debates. Mr. Trump has threatened in the past to skip debates, but his remarks Tuesday night were his strongest statement yet that he would not participate.

Fox News said Mr. Trump’s refusal to debate his rivals was “near unprecedented.”

“This is rooted in one thing – Megyn Kelly, whom he has viciously attacked since August and has now spent four days demanding be removed from the debate stage,” the network said in a statement.

“Capitulating to politicians’ ultimatums about a debate moderator violates all journalistic standards, as do threats, including the one leveled by Trump’s campaign manager Corey Lewandowski toward Megyn Kelly,” the statement continued. “In a call on Saturday with a Fox News executive, Lewandowski stated that Megyn had a ‘rough couple of days after that last debate’ and he ‘would hate to have her go through that again.’ Lewandowski was warned not to level any more threats, but he continued to do so. We can’t give in to terrorizations toward any of our employees.”

The drama capped an increasingly heated war of words involving Mr. Trump, Ms. Kelly and Fox News as the debate neared and as Mr. Trump found himself facing the prospect of being questioned by her again before a national audience.

Mr. Trump had lashed out against Ms. Kelly back in August over her questioning of him at a debate, when she asked if previous remarks he had made were a “part of the war on women.”

The clash between Mr. Trump and Fox News reignited over the last few weeks, as the network set out to defend Ms. Kelly from the repeated criticisms of Mr. Trump.

Mr. Trump threw the first punch, tweeting that Ms. Kelly had a “conflict of interest” and “should not be allowed to be the moderator of the next debate.”

The network was quick to respond, saying: “Megyn Kelly has no conflict of interest. Donald Trump is just trying to build up the audience for Thursday’s debate, for which we thank him.”

He continued to accuse Ms. Kelly of journalistic malpractice and bias, before taking the criticisms personal in an interview with CNN on Monday.

“I don’t like her. She doesn’t treat me fairly. I’m not a big fan of hers at all,” Mr. Trump said in the interview.

Fox responded, again defending Ms. Kelly: “Sooner or later Donald Trump, even if he’s president, is going to have to learn that he doesn’t get to pick the journalists — we’re very surprised he’s willing to show that much fear about being questioned by Megyn Kelly.”

But what seemed to really draw the ire of Mr. Trump was two statements put out by the network Tuesday morning. In one, Roger Ailes, the chairman and chief executive of Fox News, personally weighed in, saying in a statement that “Megyn Kelly is an excellent journalist and the entire network stands behind her — she will absolutely be on the debate stage on Thursday night.”

A statement from the network was even harsher, invoking President Vladimir V. Putin of Russia and Iran’s supreme leader: “We learned from a secret back channel that the Ayatollah and Putin both intend to treat Donald Trump unfairly when they meet with him if he becomes president — a nefarious source tells us that Trump has his own secret plan to replace the Cabinet with his Twitter followers to see if he should even go to those meetings.”

Mr. Trump alluded to those two statements in a news conference on Tuesday before an event here, calling them “wise guy” and adding, “I said bye-bye.” He did not mention his decision not to attend the debate at his two rallies in Iowa.

Mr. Lewandowski said that instead of attending Thursday’s debate, Mr. Trump would hold an event in Iowa, raising money for wounded soldiers in the state.

Mr. Trump’s main rival in various polls, Senator Ted Cruz of Texas, accused Mr. Trump of chickening out of the debate.

“If Donald is afraid to defend his record, that speaks volumes,” he said, before alluding to Mr. Putin: “I promise you, Putin is a lot scarier than Megyn Kelly.”

Mr. Trump made his remarks about skipping the event right after the Fox News debate moderator Bret Baier announced the debate lineup on the program “Special Report.”

Another of Mr. Trump’s rivals, Senator Rand Paul of Kentucky, will rejoin the top tier of candidates on the stage Thursday, Fox News said, meaning eight candidates qualified to share the stage for the prime-time debate. Mr. Trump was to stand in the middle because he leads in polls nationally and in New Hampshire, and he is battling for the lead in Iowa polls against Mr. Cruz.

For Mr. Paul, who chose not to participate in the undercard debate this month after failing to make the cut for the later debate, the return to the main stage reflects a recent rise in his poll numbers, particularly in New Hampshire. His campaign sought to capitalize on the good news, fund-raising on Twitter after his inclusion was announced on Fox News.

Rounding out the top-tier field on Thursday are Gov. John Kasich of Ohio, Ben Carson, former Gov. Jeb Bush of Florida and Gov. Chris Christie of New Jersey.

And former Gov. Jim Gilmore of Virginia, the Republican presidential candidate who has failed to qualify for the undercard debate every time except during Fox News’s first debate, will return to the debate stage, participating in the earlier debate with Carly Fiorina, former Gov. Mike Huckabee of Arkansas and former Senator Rick Santorum of Pennsylvania.

Donald Trump, in Feud With Fox News, Shuns Debate Mr. Trump and Fox News faced off again, this time over the anchor Megyn Kelly’s inclusion as moderator for Thursday’s debate.