National Journal reported that the super PAC told the Federal Election Commission it had spent $138,000 on a TV ad buy opposing Walsh. This is the first time American Crossroads has waded into a Senate race this election cycle.
Walsh, who was appointed by Gov. Steve Bullock (D) to serve out the remainder of Democratic Sen. Max Baucus' term, is already running to defend his seat in November's election. Baucus was confirmed Thursday as the next U.S. ambassador to China, and Walsh will be sworn in Tuesday.
He will face former Lt. Gov. John Bohlinger in Montana's Democratic primary in June. Rep. Steve Daines is the favored candidate on the Republican side in a race that could make or break Democratic control of the Senate.
Rove's group was criticized in 2012 for failing to help Republicans win crucial Senate races, much less oust President Barack Obama.