WASHINGTON (Reuters) - U.S. presidential candidate Donald Trump on Wednesday escalated a fight with Fox News, using the word “bimbo” in a derogatory tweet about anchorwoman Megyn Kelly after pulling out of a debate only days before the first nominating contest of the 2016 campaign.

Trump on Tuesday withdrew from the televised encounter, scheduled for Thursday night in Des Moines, Iowa, in irritation at host Fox News FOXA.O for allowing Kelly to moderate after her questioning angered him in a debate last year.

The real estate magnate, who is the Republican front-runner to win the nomination for the Nov. 8 presidential election, followed up with another round of insults on Wednesday.

“I refuse to call Megyn Kelly a bimbo, because that would not be politically correct,” he wrote on Twitter. “Instead I will only call her a lightweight reporter!”

In a later interview on Fox News’s “The O’Reilly Factor,” he told host Bill O’Reilly that she was “highly overrated.”

“I have zero respect for Megyn Kelly. I don’t think she’s very good at what she does,” Trump said.

Trump’s Republican presidential rivals were quick to criticize the former reality TV star, with U.S. Senator Ted Cruz of Texas blistering him in a series of tweets and accusing him of “trembling at being questioned by Kelly.”

At an event in West Des Moines, Iowa, Cruz openly mocked Trump, calling him a “fragile soul” and “gentle,” and renewed his offer to debate Trump one-on-one Saturday evening.

“It’s not that he’s afraid of me,” Cruz said to the crowd. “He’s afraid of you. He doesn’t want to answer questions from the men and women of Iowa about how his record doesn’t match what he’s selling.”

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Another Republican hopeful, U.S. Senator Rand Paul of Kentucky, told Fox News that he welcomed Trump’s absence from the debate stage because it means “we don’t have to put up with a lot of empty blather and boastfulness and calling people names.”

Trump’s decision means the last televised debate before Monday’s Iowa caucuses - which kick off the state-by-state nominating race to choose candidates for the presidential election - will not feature the man who has dominated the Republican race for months and leads many opinion polls.

It was seen as a bold gamble.

‘A RISKY MOVE’

Trump has said that in lieu of his debate participation he will hold a fundraiser for veterans. It will be held at Drake University in Des Moines and begin at the same time Thursday as the Fox debate, according to an invitation his campaign circulated Wednesday evening.

“It’s a risky move; it’s very high profile,” said Craig Robinson, a former Iowa state party official. “But I’m not sure it will really change anyone’s mind about Trump.”

Trump has been feuding with Fox News since the network hosted the first Republican debate in August, in which Kelly asked Trump about his treatment of women, prompting a stream of insults from the candidate and complaints he was not being treated fairly.

He did not renew his attacks on Kelly during an evening rally in Gilbert, South Carolina, but told supporters to watch a rerun of the Fox interview, which he called a “tough interview,” when they got home.

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Cruz, Trump’s main rival in Iowa, used the hashtag #DuckingDonald to make fun of Trump for ducking out of the debate and tweeted a mocked-up picture of Trump’s head on Scrooge McDuck’s body sitting on a pile of money.

Cruz, a conservative and a debating champion in college, tweeted a link to “duckingdonald.com,” which asked visitors to sign a petition in favor of Cruz and Trump having a side debate.

New Jersey Governor Chris Christie, another Republican presidential contender, described Trump’s decision at such a crucial time as “a big mistake” that calls into question his ability to be president.

“Anytime you get a podium and a microphone and 15-20 million people watching in an election campaign, you should take it,” Christie told Boston Herald Radio.

Another Republican candidate, U.S. Senator Marco Rubio of Florida, said he wanted to focus on keeping the party united in order to beat Hillary Clinton, the former secretary of state, if she becomes the Democratic nominee.

“These kinds of theatrics by Ted Cruz and Donald Trump are an entertaining sideshow, but they have nothing to do with defeating Hillary Clinton,” Rubio said.

Not every candidate was convinced Trump would follow through on his pledge to stay away.

“He apparently is not going to come to the debate, although I’ve got a $20 bet he’ll show up,” former Florida Governor Jeb Bush said at a town hall meeting in response to a question.

“Poor little Donald, being treated unfairly,” he said.

Trump’s campaign manager, Corey Lewandowski, said Trump was not afraid to debate his rivals or take reporters’ questions. He added that Trump would be happy to debate Cruz if the contest, in which 12 Republicans are vying for their party’s nomination, narrows.

“If it comes down to a two-person race, Donald Trump would be happy to debate him,” Lewandowski told ABC’s “Good Morning America” program.

Fox News, in a statement on Tuesday, said it would not "give in to terrorizations toward any of our employees," but left the door open to Trump attending the debate. The event will be co-hosted by Alphabet Inc's GOOGL.O Google.

“At the end of the day, Mr. Trump is going to have the last laugh,” Lewandowski told MSNBC.

(This version of the story corrects 17th paragraph to say Scrooge McDuck, not Donald Duck)