Douthat sees no downside:
[P]recisely because Trump’s highest goal is so transparently the perpetuation of his own celebrity, his latest attention-seeking stunt offers Romney an almost cost-free chance to repudiate a figure who’s notionally to his "right" (though in reality lacks any ideological commitment whatsoever) without risking any kind of sustained conservative revolt. Trump isn’t Rush Limbaugh or Sarah Palin: His conservatism is feigned, his right-wing fans are temporary admirers with no deep commitment to his brand or cause, and hardly anyone in the conservative media is likely to rise to his defense.
Byron York provides a window into the Romney campaign's thinking:
Romney aides believe that cooperating with Democrats and media figures who are demanding a Trump disavowal would most certainly lead to more calls for more disavowals of other figures in the future — leaving Romney spending as much time apologizing for his supporters as campaigning for president. Team Romney views it as a silly and one-sided game designed to distract voters from the central issue of the race, which they remain convinced will be President Obama's handling of the economy.