The anti-tax Club for Growth is spending $600,000 in Indiana on an ad hitting Democrat Joe Donnelly for supporting congressional earmarks.
The club lifted Indiana Republican Richard Mourdock over U.S. Sen. Richard Lugar in May's primary and has bought sporadically in Indiana since then.
Club spokesman Barney Keller said the ad will show Donnelly is "just another liberal" who supported Washington spending. Donnelly has pushed back against that argument throughout the campaign, noting most recently that he voted with Republican Speaker John Boehner 60 percent of the time.
Indiana's Senate battle has grown into the most expensive the state has ever seen.