Obama's campaign says Tuesday it had little choice but to bless the support for Priorities USA, a Democratic super PAC led by two former Obama aides.
Campaign officials say they couldn't "unilaterally disarm" in the face of hundreds of millions of dollars expected to be raised by Republican outside groups.
Obama has opposed the Supreme Court's 2010 decision in the Citizens United case that stripped away some limits on campaign contributions. The super PACs can't coordinate directly with campaigns but have played a major role in the Republican primary contests by funding millions in negative ads.
