American Crossroads is investing $512,000 in New Mexico, where Democrat Martin Heinrich has held a lead in polls over former Rep. Heather Wilson.
The Republican-leaning outside group is also investing in Nebraska's suddenly tighter Senate race.
American Crossroads, the group organized by Republican operative Karl Rove, is spending $420,000 on the race between Republican Deb Fischer and former Democratic Sen. Bob Kerrey.
The race has been considered a clear Republican pickup after Sen. Ben Nelson announced that he would not seek another term, but recent polls have showed a closer contest.
Nate Hodson, a spokesman for American Crossroads and Crossroads GPS, said they will unveil the ad on Wednesday.
Copyright 2012 The Associated Press.