American Crossroads, the Republican super PAC cofounded by Karl Rove, plans to welcome Montana Lt. Gov. John Walsh to the U.S. Senate with an attack ad.
Crossroads reported to the Federal Election Commission that it has spent $138,000 on a TV ad opposing Walsh, a Democrat appointed to the Senate on Friday. Walsh is running to retain the seat this fall.
American Crossroads spokesman Jonathan Collegio said the ad will begin airing next week. Walsh is expected to be officially sworn in on Tuesday.
Montana Gov. Steve Bullock announced Walsh would serve out the reminder of Sen. Max Baucus's term. Baucus was easily confirmed on Thursday to be the next U.S. ambassador to China; he had previously announced he would not seek reelection this year.
Walsh is the favorite against former Lt. Gov. John Bohlinger in the June 3 Democratic primary, but prior to his appointment, he was the underdog in the general election against Republican Rep. Steve Daines—in a race that could help determine the balance of power in the Senate.
The ad will mark American Crossroads' first in any Senate race this cycle.