Just when it appeared that outside spending was slowing, a super political action committee said Tuesday that it’s making an major ad buy in the final stretch of the race for U.S. Sen. Olympia Snowe’s empty seat in Maine.

Crossroads GPS, a super PAC co-founded by Republican Karl Rove, said Tuesday that it’s spending $335,000 for a new ad targeting independent former Gov. Angus King. In the ad, the narrator says King “blew it” by using influence on a government task force to help wind power companies like his own in Roxbury.

The ad purchase brings the total to nearly $1 million spent by Crossroads GPS attacking King, who is leading Republican Secretary of State Charlie Summers and Democratic state Sen. Cynthia Dill in polling.

The National Republican Senatorial Committee has stopped spending in Maine. The same is true for the Democratic Senatorial Campaign Committee, which has been attacking Summers.

King’s campaign attacked the Crossroads ad, saying King once was a member of the Ocean Energy Task Force, which focused on deep-water wind projects, while King’s company developed projects on land.

“We know Karl Rove knows zero about Maine or Angus’ record, and his only agenda is to advance partisan interests that are splitting the country. But to not understand the difference between land and sea is surprising,” said Crystal Canney, King’s spokeswoman.

The Summers campaign said the late spending by Rove’s Super PAC shows that the race is not out of reach for Republicans.

“The fact that Crossroads has locked in for the final week with a new ad should reinforce the fact that this race continues to be seen as a winnable race for Charlie,” said Lance Dutson, campaign manager for Summers, who said there’s been an uptick in enthusiasm for Republicans.

Since late July, there has been an advertising free-for-all, with conservatives attacking King as a tax-and-spend liberal and political insider and Democrats linking Summers with the tea party while largely ignoring their own candidate, Dill.

Americans Elect, meanwhile, has been airing ads on King’s behalf, thanks to $1.75 million in contributions from New York Mayor Michael Bloomberg, Americans Elect founder Peter Ackerman and San Francisco investment firm founder John Burbank.

Jim Melcher, political science professor at the University of Maine at Farmington, said negative advertising was to be expected in a race for an open Senate seat, especially in a state where media advertising is cheap relative to bigger states. But he was surprised by the volume of attack ads.

He also said he was surprised Crossroads was still attacking King when a poll earlier this month showed King with a 26-point lead. “They must still think there’s an opportunity there.”