American Crossroads and its sister group Crossroads GPS are launching a $4.6 million ad buy aimed at Democratic Senate candidates in six states.

American Crossroads is going on the air in Nebraska against Bob Kerrey, Nevada against Rep. Shelley Berkley and Virginia against Tim Kaine. Crossroads GPS, a sister arm that can raise unlimited funds from anonymous donors, is going up against Sen. Claire McCaskill in Missouri, against Heidi Heitkamp in North Dakota and against Sen. Sherrod Brown in Ohio.

Most of the spots tie candidates to President Obama and his health-care legislation. But the Nevada ad is harsher, hitting Berkley over allegations that she used her office to help her husband’s kidney care business.

Berkley, who is challenging Sen. Dean Heller (R), faces a House Ethics Committee investigation. She said last weekend that the race will “not pivot” on the issue.

Combined with ads last week in New Mexico, Indiana and Montana, it’s a $5.7 million campaign.

Republican outside groups have been pummeling Democratic senators and Senate candidates for months. Some Democrats have tried to make an issue out of the millions of dollars coming in from out of state, but its hard to engage voters on campaign finance issues.