MARSHALLTOWN, Iowa — Donald J. Trump and Fox News, the candidate who has reordered the Republican presidential race and the cable network of choice for many of the party’s voters, stared each other down on Tuesday over his demand that the news anchor Megyn Kelly be dumped from moderating Thursday’s debate, the last before Monday’s caucuses.
The network did not blink. So Mr. Trump walked.
Mr. Trump’s announcement here that he would “probably,” or would “most likely,” or was “pretty close to” irrevocably planning to skip the debate — an aide put it more directly — created a gaping uncertainty at the center of the Republican nominating contest just as it was formally about to begin in Iowa.
It was the most intense confrontation yet between two mutually dependent but increasingly antagonistic powerhouses of media and politics. Mr. Trump, who has made the presidential race into a riveting television spectacle, was overtly exploiting the ratings leverage his candidacy has created to try to bend Fox News to his will.
“Let’s see how much money Fox is going to make on the debate without me,” he said at a news conference here.
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.
On her program Tuesday night, Ms. Kelly observed that “what’s interesting here is Trump is not used to not controlling things, as the chief executive of a large organization.”
“But the truth is, he doesn’t get to control the media,” she added.
Mr. Trump’s animus toward Ms. Kelly dates to August, in the first presidential primary debate, when she questioned him about his past comments denigrating women. Afterward, he suggested that Ms. Kelly had been angry at him, so much so that she had blood pouring out of her “wherever” — a remark many saw as a reference to menstruation.
In the months since, Mr. Trump has repeatedly criticized Ms. Kelly as a “third-rate” reporter. And as Thursday’s debate approached, Mr. Trump began disparaging Ms. Kelly as if he were a prizefighter promoting a rematch. He called her dishonest, accused her of bias and a conflict of interest, and said flat-out that he did not like her.
On Monday, Fox News responded to Mr. Trump, tauntingly saying it was “surprised he’s willing to show that much fear” about being questioned by Ms. Kelly. And on Tuesday, after the network’s president, Roger Ailes, declared that Ms. Kelly would “absolutely be on the debate stage,” the network issued yet another taunting statement, this one mocking two of Mr. Trump’s go-to rhetorical crutches.
“We learned from a secret back channel,” the statement said, “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.”
At that, Mr. Trump told Mr. Ailes and his network, effectively, to take a flying leap.
“With me, they’re dealing with somebody that’s a little bit different,” he said of Fox. “They can’t toy with me like they toy with everybody else. So let them have their debate, and let’s see how they do with the ratings.”
Mr. Trump made his remarks right after Bret Baier, one of the network’s three moderators, announced the lineup for the debate on the program “Special Report.”
One of Mr. Trump’s rivals, Senator Rand Paul of Kentucky, will rejoin the top tier of candidates on the stage for Thursday’s event, the network said, meaning eight candidates qualified to share the stage for the prime-time debate. Mr. Trump was to remain at center stage, by virtue of his lead in the polls.
His absence from the debate stage could strip the event of much of its appeal for many viewers who have followed the presidential contest as much as a source of entertainment as for its political importance.
“Obviously we would love all of the candidates to participate,” said Sean Spicer, a spokesman for the Republican National Committee, “but each campaign ultimately makes their own decision what’s in their best interest.”
The tepid response was the latest instance in which the party has tried not to antagonize Mr. Trump, even as he engages in behavior that many Republican donors and operatives and some committee members consider destructive.
Major presidential candidates have skipped primary debates in the past, most notably in 1980, when Ronald Reagan opted not to debate George Bush before the Iowa caucuses, rankling some in Iowa. Mr. Reagan eventually lost the caucuses. And George W. Bush passed on the first few debates in 2000 but rebounded to win the nomination after a lopsided defeat in New Hampshire.
It was not the first time that Mr. Trump, who holds a wide lead in national polls and a slender one in Iowa, has threatened to sit out a debate. At one point he demanded that CNN donate $5 million to aid wounded veterans in return for his participation. His logic then was the same as now: The ratings stem from his presence. But CNN officials declined, essentially calling Mr. Trump’s bluff, and he participated.
This time, though, his campaign manager, Corey Lewandowski, said there would be no backing down Instead of attending the debate, he said, Mr. Trump would hold a fund-raiser in Iowa for wounded war veterans.
Mr. Lewandowski did not respond to a message asking the chances that his boss would change his mind.
In his news conference, held in a high school weight room before a rally in Marshalltown, Mr. Trump left open the possibility that he might still attend, and allowed that it could fairly be called a flip-flop by critics. Still, the controversy ate away yet another news cycle in a dwindling number of them before the Iowa caucuses, in which Mr. Trump is in a close contest with Senator Ted Cruz of Texas.
Mr. Cruz, appearing on the radio show of the conservative host Mark Levin, said Mr. Trump’s withdrawal from the debate “reflects a lack of respect for the men and women of Iowa,” and challenged Mr. Trump to a one-on-one debate on Mr. Levin’s show.
“He can name his own moderator,” Mr. Cruz said.
Other rivals pounced, too. On Twitter, Jeb Bush highlighted a response by the conservative writer Erick Erickson: “So Donald Trump can’t handle tough questions from Megyn Kelly, but he’ll be able to handle Hillary Clinton. Hahahahahahahahahahahahahahahaha.”
“Exactly,” Mr. Bush added tersely, in his own Twitter message.