There have been countless ads in dozens of races this cycle in which a Republican candidate or group takes a Democrat to task for supporting Obamacare. In the Nebraska Senate race, you'll find something much rarer: A Republican hitting another Republican over the law.

Former state treasurer Shane Osborn (R) is up with a new TV ad hitting Midland University President Ben Sasse (R) for being, in Osborn's portrayal, too soft on Obamacare.

"Sasse called Obamacare, quote, 'an important first step.' He said Obamacare’s mandate might be a good idea. He said it won’t be repealed. He gave up," says Osborn, who contrasts himself with his opponent.

The "important first step" reference is to remarks Sasse made in 2010 speech. He also criticized the law in the speech, which is not mentioned in Osborn's ad. Sasse's campaign said the ad was "dishonest."

"Here in Nebraska, honesty matters but Shane Osborn's dishonest attack is proof that he is willing to do and say anything to get elected. Nebraskans know the truth: No one has fought ObamaCare harder than Ben Sasse," said Sasse spokesman Tyler Grassmeyer.

While the full context of Sasse's remarks paint a clearer picture of the spirit of his words, the ad is a reminder that Republicans must be very careful how they speak about Obamacare in public setting, lest they fall victim to an attack from a fellow Republican at some point.

We saw this play out in the Georgia Senate race  this year when Rep. Paul Broun (R) went after Rep. Jack Kington (R) with a web ad hitting him for saying it's not "the responsible thing to do" for Republicans to let Obamacare fail without trying to fix it.

The Nebraska primary has heated up in recent weeks. Sasse recently stumped with Sarah Palin and Sens. Ted Cruz (R-Tex.) and Mike Lee (R-Utah). The primary is May 13.

Osborn's campaign is spending about $200,000 to air the ad in the state. It was first reported by Hotline On Call.

Obamacare is the issue that animates the Republican base this cycle. So don't expect to only see it raised in general elections against Democrats.