The former Massachusetts governor joins U.S. Rep. Ron Paul of Texas and former Utah Gov. Jon Huntsman on the sidelines for the Dec. 27 debate, being sponsored by Newsmax and ION Television, Politico reported.
That leaves former House Speaker Newt Gingrich of Georgia and former U.S. Sen. Rick Santorum of Pennsylvania as the only two confirmed participants. U.S. Rep. Michele Bachmann of Minnesota and Texas Gov. Rick Perry had yet to disclose their intentions, the Washington publication said.
Romney, who trails Gingrich in the polls, told Fox News Channel he was passing up the debate because his plate is full with getting ready for other debates already on his schedule and "doing the political work" a presidential campaign dictates must done.
"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 -- who this year toyed with the idea of running for the GOP nomination and recently put forth the prospect of a possible independent candidacy -- said in a statement. "But I can also understand why Gov. Romney decided not to do it."
Romney said he declined the debate invitation directly, Politico said.
"I spoke with Donald Trump earlier today and indicated that we just can't make this debate. We're going to focus on the other two we've got and on some campaigning," he said.