(CNN) - Donald Trump's Republican presidential debate took another hit Wednesday when White House hopeful Mitt Romney said he wouldn't participate in the event.
Romney is now the third candidate to decline Trump's invitation to participate in the event. Former Utah Gov. Jon Huntsman and Texas Rep. Ron Paul both said earlier this week they were skipping the debate, questioning Trump's seriousness as a political moderator.
Texas Gov. Rick Perry, former Pennsylvania Sen. Rick Santorum and former House Speaker Newt Gingrich have all said they'll be on the stage at the debate, which is slated for Dec. 27. Minnesota Rep. Michele Bachmann has yet to say whether or not she'll participate.
Speaking to reporters Wednesday in Arizona, Romney said the upcoming debate schedule was so crowded he had to say no to Trump.
"We've had requests for quite a few debates, we decided on two in December. The 10th and 15th and we're not adding any others in December," Romney said. "We'll look at January's request. I think we have ten requests for January debates. We'll probably do two or maybe three in January."
The former Massachusetts governor said he had spoken with the reality show host and businessman Wednesday morning to submit his regrets.
"We've already set our calendar in December and I communicated to Mr. Trump that that schedule is completed," Romney said.
In a statement to CNN, Trump cited Romney's declining poll numbers, but said he understood the candidate's predicament.
"It would seem logical to me that if I was substantially behind in the polls, especially in Iowa, South Carolina and Florida, I would be participating in the debate," Trump said. "But, I can also understand why Governor Romney decided not to do it."