The Republican super PAC American Crossroads and its affiliated nonprofit Crossroads GPS are kicking off a $9 million+ ad spending spree in four key races in the battle for the U.S. Senate majority.

The sizable summer campaign launches in North Carolina Tuesday, the group said, where Crossroads GPS has purchased nearly $3.6 million worth of air time. Republican Thom Tillis recently won his party's nomination there and faces Sen. Kay Hagan (D).

Crossroads GPS will  hit the airwaves in Alaska later this month, where air time is cheaper than most states. The group is spending $900,000 there, in addition to more than $3 million combined in Arkansas and Colorado. American Crossroads will be on the air with commercials in Alaska and Arkansas next month backed by nearly $1.8 million.

In all four states, Democratic senators are running for reelection.

Crossroads was co-founded by Republican strategist Karl Rove. Democrats took aim at Rove in a statement responding to the ad buys.

“Karl Rove’s dark money special interest group spent $63 million last cycle in contested Senate races and the effort was a colossal failure, as they lost almost every single one," said Democratic Senatorial Campaign Committee spokesman Justin Barasky.

News of the buys was first reported by Politico.