The radio ad mimics a back-and-forth between two Nebraska voters, with the first informing the second that "Bruning's record is terrible."

"So Jon Bruning praised the Obama stimulus, and doubled his own office spending?" says the first voter.

"Yep," says the second voter.

"That's not what we need in Washington," replies the first voter.

It's the first major investment by the Club for Growth in the race to replace retiring Sen. Ben Nelson (D-Neb.), but the group made clear in March it was backing state Treasurer Don Stenberg (R) in the primary.

The group has been joined in its support for Stenberg by other fiscally conservative groups, including Sen. Jim DeMint's (R-S.C.) Senate Conservatives Fund, which has gone on the air twice already with ads supporting Stenberg.

Stenberg is running behind Bruning in the polls, but his support from deep-pocketed conservative groups has helped him give Bruning a run for his money — and to make the case that he's the bona fide fiscal conservative in the race.

Meanwhile, another outside group, Americans for Prosperity, is also on the air in Nebraska with ads targeting former Sen. Bob Kerrey (Neb.), the Democratic recruit to replace Nelson. The new ad from the group founded by the billionaire Koch brothers accuses Kerrey of becoming more liberal since he left the Senate and took a job in New York.

A third GOP candidate, state Sen. Deb FischerDebra (Deb) Strobel FischerWhatever you think the Alabama special election means, you’re probably wrong The Hill's 12:30 Report Breitbart charts path for 2018 midterm races MORE, launched her first ads of the campaign on Monday, spending about $35,000 for a bio-spot touting her background as a small-business rancher.

The Hill rates this race as leaning Republican.

Watch the ad: