American Crossroads, the heavyweight super PAC run by Karl Rove, has reserved $6.2 million in Florida air time to oppose Sen. Bill Nelson, a spokesman confirms.

"He has a long record that makes him vulnerable," said spokesman Nate Hodson.

Crossroads plans to spend $70 million in Senate races across the country (Roll Call story here) and in an effort to give Republicans control of the chamber.

“Connie Mack IV is a seriously flawed candidate," said Marley Wilkes, Nelson's deputy campaign manager. "But Bill Nelson isn’t being challenged by Connie Mack IV. He’s under attack by a handful of billionaires operating in secret. And they don’t care about Florida. For them it’s all about protecting their own self-interest in Washington.”