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Priorities USA Action, the Obama super PAC, also spent $230,000 attacking Mr. Romney in Michigan, making it the first state in which the PAC has become significantly involved. The group, which has struggled to raise funds, has spent only $800,000 overall. In all cases, the Democratic group has fought for Mr. Obama’s election by encouraging Republicans not to vote for Mr. Romney.

It was unclear whether the super PAC hoped to garner anti-Romney votes from Democrats or whether it hoped only to push Republicans to the right.

AFSCME is the only union to get involved in the election so far with the exception of $145,000 in anti-Romney radio ads from the Service Employees International Union in Florida and Nevada, but unions will be major players in funding ads as the general election nears.