In Missouri, the group's ad will outline McCaskill's positive record on veterans' issues, featuring veterans Jackie Rodgers and Colin Clark, both of whom received help from the 21st Century GI Bill, which was backed by McCaskill. The ad will run in the Springfield, Mo., market. The group is buying Facebook ads as well.

In Montana, the VoteVets ad is decidedly more negative, as it attacks Rep. Denny Rehberg (R-Mont.) for voting against a bill that would have increased  funding for prosthetic research, and also for voting for a bill to raise lawmakers' pay. It will run in the Billings, Butte and Missoula markets in the state, and is backed up by additional Facebook ads.

Both McCaskill and Tester face tough races in Republican-leaning states, but missteps by McCaskill's opponent, Rep. Todd Akin (R-Mo.), have improved her chances at a win, and Tester has worked to hedge his reelection chances by taking a more centrist position with many of his votes in the Senate.