The Republican super PAC American Crossroads and its affiliate, Crossroads GPS, have launched a $4.6 million ad buy slamming President Obama and Democratic Senate candidates in close races across half a dozen states. 

American Crossroads, the GOP spending behemoth led by Karl Rove, is airing ads in Virginia, Nebraska and Nevada targeting Senate candidates Tim Kaine, the former governor of Virginia, Bob Kerrey, the former governor of Nebraska, and Nevada Rep. Shelley Berkley.

Meanwhile, Crossroads GPS will air ads targeting Obama in Missouri, North Dakota and Ohio, states that also have close Senate races. The ads from both groups will air for two weeks.

The ad attacking Kaine highlights his ties to Obama and claims that Kaine's national aspirations outweigh his commitment to Virginia.

Questioning why Kaine would put his national aspirations first, the ad shows a clip of Kaine saying, "The answer is to serve the president."

"These liberal candidates are unfortunate examples of what's wrong in Washington — politicians who put their own personal agendas ahead of the people they represent," Nate Hodson, a spokesman for American Crossroads, said in a statement on the ad buy. "We can't fix the broken economy and national debt by electing these self-serving liberal politicians to the U.S. Senate."

The ad buy follows a $1.1 million offensive launched last week in three other states with close Senate races: New Mexico, Indiana and Montana.