Tis the season for slinging some mud and few campaigns in Nebraska are as heated as the one between Deb Fischer and Bob Kerrey.

Kerrey recently launched an ad accusing Fischer of an attempted land grab back in the 90's.

Tuesday, Fischer shot back, calling Kerrey desperate in a press conference.

And while some candidates say they're running a clean, non-negative race, their supporters, like super pacs, are still able to produce damaging ads.

So how do you tell the difference?

UNL political science professor John Hibbing says, "I know we get bored with hearing that 'I'm so and so and I approve this ad', but that's the good way to tell, right? Because if you approve this ad then that means it's yours and there's no way you can disavow what's in that ad."

Super pacs are not allowed to coordinate with a candidate, but they can create ads attacking that candidate's opponent.

Experts say many voters retain the information in negative ads the best, which is why they are a popular campaign tactic.