MARSHALLTOWN,
Iowa — Moments after Fox News announced the lineup for Thursday’s
Republican debate, Donald J. Trump said Tuesday 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 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.

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, Senator Ted Cruz, 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, 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 earlier 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.

And Jim Gilmore, 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, Mike Huckabee and Rick Santorum.