Worried about the flood of unlimited -- and often undisclosed -- dollars streaming into political action committees that are backing Republicans, President Obama's campaign is changing its tune -- which had been to oppose the committees.

In a late night email to supporters, Obama campaign manager Jim Messina says it has decided to back Priorities USA, the super PAC run by former WH spox Bill Burton.

Messina called the Republican super PACs, a "real risk," noting that in 2011 the committee supporting Mitt Romney raised $30 million from less than 200 contributors. "Ninety-six percent of what they've spent so far, more than $18 million, has been on attack ads," he said.

Obama vociferously opposed the Citizens United Supreme Court decision that opened the door to super PACS, but Messina argues that "this cycle, our campaign has to face the reality of the law as it currently stands.

"With so much at stake, we can't allow for two sets of rules in this election whereby the Republican nominee is the beneficiary of unlimited spending and Democrats unilaterally disarm."