The ad shows footage of Thompson adamantly refusing to release the returns. "'The question is, am I going to release my tax returns? My answer is no. No," he says. "The answer is N-O. What part don't you understand?"
The ad is Baldwin's first since the Republican primary was decided last Tuesday. Thompson remains fairly popular with independents in the state and has led her in polls, but Democrats believe if they can convince voters he cashed in with lobbyists in the past few years, they can defeat him.
The attack could gain some traction: The well-read Milwaukee Journal-Sentinel released an editorial on Tuesday calling on Thompson to release his returns.