An independent Democratic group led by former aides to President Obama launched the first ad of the 2012 cycle with an attack Friday on former Massachusetts Gov. Mitt Romney (R).

Priorities USA Action, the liberal group founded by White House alumni Bill Burton and Sean Sweeney, launched an ad to play in heavy rotation on television in South Carolina, where Romney's visiting this weekend.

The ad casts Romney as caught between former House Speaker Newt Gingrich, another Republican presidential candidate who'd caught flak this week for criticizing House Republicans' 2012 budget, and South Carolina Gov. Nikki Haley (R), who backs the budget drafted by Rep. Paul Ryan (R-Wis.), including its proposed transformation of Medicare.

"With Mitt Romney, you have to wonder — which page is he on today?" the ad asks, seizing on suspicions that the putative Republican front-runner could shift positions on the Ryan budget, for which he's expressed support.

The ad is significant because it represents the first foray by Priorities USA Action, a so-called "Super PAC" that isn't bound by limits on donations, into the presidential cycle, where it's expected to play a big role.

Romney's camp cast the ad as nothing more than a negative attack by loyalists to Obama.

"President Obama’s first campaign ad is an attack ad. President Obama and his team are desperate to change the subject to anything other than jobs and the millions of Americans out of work," said Andrea Saul, a spokeswoman for Romney's exploratory committee.

The ad also manages to take a shot at Gingrich, who's hoping to play for primary votes in South Carolina, by seizing on remarks he made last weekend that were critical of the Ryan budget. Gingrich has since said those comments were a "mistake."

Priorities USA and Priorities USA Action mirror the twin supergroups created by Karl Rove and Ed Gillespie, two veterans of the Bush administration, that greatly benefited Republicans in the 2010 elections. Democrats had assailed those groups, American Crossroads and Crossroads GPS, and demanded that they disclose their donors.

Crossroads never complied, and now some Republicans are wondering whether Priorities USA will reveal its funding sources, especially since the White House said it believes Priorities USA should do just that.

"We're not going to go any further than Karl Rove does," Burton said Friday on MSNBC.