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“It’s not so much that Democrats view Romney as inevitable, it’s that he’s the most likely to beat Obama,” Mr. Walker said. Democrats are “drooling at the prospect of the party throwing Romney over and flocking to Gingrich.”

The Romney group’s attack on Mr. Gingrich throws a wrench in the conviction of eventuality, and the pause in the Obama group’s attack could indicate it is standing back to let the Republicans bloody themselves.

“By running a blisteringly negative campaign against Newt Gingrich, Romney’s Restore Our Future may well be hurting Romney by underscoring doubts and concerns conservatives already had against him,” Mr. Burton said in an email to The Washington Times.

“We’ve focused on Romney because he’s been the most likely nominee thus far. But Newt Gingrich will get his turn in the barrel soon enough.”

The fact that Mr. LaBolt even responded to the Restore our Future ad buy shows that Mr. Obama views Mr. Romney with a trepidation that he does not reserve for other candidates.

Asked to respond to a Rick Perry ad Friday, spokesman Jay Carney would say only: “I think that I’ll limit my comment on the struggling state of some presidential campaigns.”