Texas Gov. Rick Perry and U.S. Rep. Michele Bachmann of Minnesota Thursday said they would not take part -- leaving only former U.S. House Speaker Newt Gingrich and former U.S. Sen. Rick Santorum of Pennsylvania to participate in the debate, scheduled for Dec. 27.
Former Massachusetts Gov. Mitt Romney, Rep. Ron Paul of Texas, and former Utah Gov. Jon Hunstman had already said they would not take part in the debate, sponsored by two conservative media outlets, Newsmax and ION Television.
Bachmann told Fox News Channel Thursday she would not be in the debate because of concern that Trump has already decided which presidential candidate he will endorse.
"It suggests the idea of bias and I just don't know if that that's necessarily the right format," she said.
Perry's campaign manager said the Texas governor "respectfully declined," the New York Daily News reported.
Romney Tuesday joined Paul and Huntsman on the sidelines, telling FNC he would pass 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.