PORTLAND, Maine — Just when it appeared that outside spending was slowing, a super political action committee said Tuesday it is making another major ad buy in the final stretch of the race for US Senator Olympia Snowe’s empty seat in Maine.

Crossroads GPS, a super PAC cofounded by Republican Karl Rove, said Tuesday that it is spending $335,000 for a new ad targeting independent former governor Angus King. In the ad, the narrator says King ‘‘blew it’’ by using influence on a government task force to help windmill companies like his own former wind company in Roxbury.