Priorities USA also plans to continue a Spanish-language campaign in Colorado, Florida and Nevada in conjunction with the Service Employees International Union. The groups plan to spend at least $4 million on that campaign.

Priorities announced last month that it raised $6 million in June, the largest ever single-month haul for the group. Donors included actor Morgan Freeman, who gave $1 million.

But that total was dwarfed by Restore our Future, the pro-Romney super-PAC that quadrupled its haul between May and June and raised a record $20 million. Half that total came from a $10 million single-check donation from casino mogul Sheldon Adelson, who has spent heavily to support Republicans this election cycle.

Other Republican groups — including Karl Rove's American Crossroads and Crossroads GPS — also look to be prominent players, having already spent tens of millions on advertising.