The big-spending GOP outside group Crossroads GPS has bought $7.2 million in airtime in five key Senate states, an ad-tracking source told The Hill.

The ads will begin to air this Wednesday in Missouri, Montana, Nevada, North Dakota and Virginia — all key swing states The Hill rates as "toss-ups." The GOP needs to gain a net of four seats for Senate control.

The group did not immediately respond to requests for comment.

The Democratic Senatorial Campaign Committee (DSCC) fired back at the group and its founder, top GOP strategist Karl Rove, pointing to a Federal Election Commission complaint it has filed that argues the group should have to disclose its donors.

"Karl Rove is spending millions propping up George Allen, Dean HellerDean Arthur HellerSenate Dems build huge cash edge in battlegrounds Jacky Rosen hits Dean Heller over health care in first negative ad GOP moderates hint at smooth confirmation ahead for Kavanaugh MORE, Denny Rehberg, Rick Berg, John Brunner, Sarah Steelman and Todd Akin because he knows that these Republicans will be a rubber stamp for the special interests that are lining Rove's pockets," said DSCC spokesman Matt Canter. "What's worse is that Rove is blatantly thumbing his nose at election laws and allowing billionaires and corporate special interests to avoid federal election law with their anonymous, shadowy donations."