The email goes on to ask supporters to donate to the campaign, and directs recipients to a fundraising website bemoaning "slimy attack ads."

The Obama campaign has blasted Republicans in the aftermath of a New York Times story revealing details of the plan, which would have involved some $10 million in advertising expenditures by a super-PAC run by billionaire Chicago Cubs owner Joe Ricketts.

The Romney campaign was quick to distance itself from the controversial proposal, as were other GOP leaders.

Making his first public comments on the matter following a campaign event in Jacksonville, Fla., Romney said personal attacks were the "wrong course for a PAC or a campaign."

"I hope that our campaigns can ... be about the future and the issues and a vision for our future. I've been disappointed in the president's campaign to date, which is focused on character assassination," Romney said. "I just think we're wiser to talk about the issues of the day, what we'd do to get America working again, talk about our respective records."

But the Obama campaign said the presumptive Republican nominee did not do enough to distance himself from the proposal.

“Today, Mitt Romney had the opportunity to distance himself from his previous attempts to inject the divisive politics of character assassination into the presidential race. It was a moment that required moral leadership, and once again he didn’t rise to the occasion. Throughout the course of the campaign, he has repeatedly refused to stand up to the most extreme voices in the Republican Party. If this is the ‘leadership’ he has shown on the campaign trail, what can the American people expect of him as president of the United States?” said Obama spokesman Ben LaBolt.