Longtime Indiana Senator Dick Lugar is trailing state Treasurer Richard Mourdock by five points in the closely watched Republican primary, according to a new poll on behalf of Citizens United.The survey shows Mourdock leading with 44 percent and Lugar at 39 percent, with nearly 17 percent of voters still undecided. Lugar supporters point out that Citizens United is backing Mourdock in the May 8 primary, however.Both candidates have seen their favorable numbers plummet and the nastiness is expected to increase with less than two weeks left.From Politico: The Club for Growth has just reserved another $421,000 of TV time to batter Lugar, charging in a new spot that he is "clinging to power, throwing mud." Meanwhile, the pro-Lugar American Action Network is on the air with an ad attacking Mourdock’s “reckless” mismanagement of state money as treasurer.In recent days, the Lugar campaign has also launched a series of attacks against the Tea Party challenger, claiming Mourdock failed to report several owned properties while highlighting that he once supported the Fairness Doctrine.UPDATE:The Lugar campaign has responded to the Citizens United poll, calling it "non-credible" and "pro-Mourdock", adding their internal surveys show the incumbent ahead. "All along, the Mourdock campaign has been fueled by special interests from outside the State of Indiana—and this poll is just another example. Our internal polling shows Dick Lugar is, was and will remain in the lead. Our polling also shows Richard Mourdock's unfavorable ratings growing rapidly," says Lugar campaign manager Emily Krueger. "The more Hoosiers learn about Mourdock, the less they trust him. At the end of the day, this race will be decided by Hoosiers and not by D.C.-based special interest polls."