The Donald Trump-moderated debate planned for Iowa at month's end may be missing one key ingredient: candidates.

Mitt Romney on Tuesday added his name to the list of Republican presidential hopefuls who are deciding against sharing a stage with the brash media mogul, saying his schedule was just too tight.

"No, I'm not participating in that," Romney said in yet another appearance on Fox News Channel on Tuesday. "I spoke with Donald Trump earlier today and indicated we just can't make it."

How did the Donald react? Romney said he "understood my perspective and wished me well."

To date, only Newt Gingrich and Rick Santorum have signed on to the Dec. 27 debate, sponsored by conservative media group Newsmax and Ion Television.

Also Tuesday, the American Conservative Union, which hosts the annual CPAC gathering, joined as an additional sponsor, and "strongly encouraged" all the candidates to participate.

Ron Paul and Jon Huntsman Jr. had declined. Paul has mocked Trump's self-appointed status as a GOP kingmaker; Huntsman's camp jabbed that the voters deserved a serious discussion of the issues.

Rep. Michele Bachmann, speaking on CBS earlier Tuesday, said she still hasn't decided, but sounded less than eager about the prospect.

"One concern was he said ... he's already leaning toward a candidate, and so, you know, there's questions about objectivity. But I have great respect for him," she said.

Exactly one week before, to be exact.

There are two Iowa debates on the books: Dec. 10 and 15. Two more debates are scheduled in January on the weekend before the New Hampshire primary.