Early Thursday morning, Trump, 69, retweeted one of his supporters who called Kelly a "bimbo" for posing for a sexy photoshoot in GQ in 2010.
"And this is the bimbo that's asking presidential questions?" the user wrote alongside pictures from the shoot that are captioned, "Criticizes Trump for objectifying women. Poses like this in GQ magazine."
Trump's campaign manager told reporters Tuesday the billionaire businessman would not be attending Thursday's GOP debate, which takes place in Iowa just days before the all-important Iowa caucuses.
In an interview with Bill O'Reilly on Wednesday, Trump did not budge from his decision, saying that he has "zero respect for Megyn Kelly," who he says is "highly overrated."
"I was not treated well by Fox," he continued. "They came out with a ridiculous PR statement, it was, like, drawn up by a child and it was a taunt and I said, you know, how much of this do you take?"
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Fox News issued a statement Tuesday night about Trump's refusal to participate in the debate.
"It should be clear to the American public by now that 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," a Fox News spokesperson said. "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. 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. Trump is still welcome at Thursday night's debate and will be treated fairly, just as he has been during his 132 appearances on FOX News & FOX Business, but he can't dictate the moderators or the questions."
On Wednesday, Trump's campaign sent out invitations to a free event in support of wounded warriors. The event will take place at Drake University in Des Moines on Thursday, and will be broadcast live by CNN.
Later Wednesday night, the super PACs supporting Ted Cruz announced that they will donate $1.5 million to veterans charities if Trump agrees to go head-to-head with Cruz in a private debate, the Washington Post reports.
"I propose a venue. Western Iowa Tech. Saturday night in Sioux City. We already have it reserved ... A two-hour, one-on-one debate," Cruz said at an anti-abortion rally in Des Moines on Wednesday. "We have a venue, we have a time, all we're missing is a candidate."
Trump has yet to respond to Cruz's proposal.
Thursday's GOP debate will air at 9 p.m. (ET) on Fox News. Trump's wounded warriors event will air on CNN, also at 9 p.m.