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 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.

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"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. Sen. 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.

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."

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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.