Washington (CNN) -

American Crossroads and its affiliated group Crossroads GPS are going up with television commercials in six more states with competitive Senate races.

The pro-Republican independent groups, co-founded by Karl Rove, last week went up with spots in three other states with crucial contests that could decide which party controls the U.S. Senate next year. And Crossroads has already dished out big bucks over the past six weeks to put up television commercials in key battleground states that are critical of President Barack Obama.

Crossroads GPS and American Crossroads announced Wednesday they will spend a combined $4.6 million to run ads on broadcast and cable television in Missouri, North Dakota, Ohio, Nebraska, Nevada, and Virginia. The hard hitting spots attack Democratic Sen. Claire McCaskill of Missouri, Heidi Heitkamp, the Democrat's Senate candidate in North Dakota, Sen. Sherrod Brown of Ohio, former Sen. Bob Kerrey, who's running for his old seat in Nebraska, Rep. Shelley Berkley, the Democrat's Senate candidate in Nevada, and former Gov. Tim Kaine, the Democrat's candidate in Virginia.

Last week the two groups started what they said would be a two week $1.12 million buy to run spots statewide on broadcast TV and cable in Indiana, Montana, and New Mexico.

Crossroads GPS is the sister organization to American Crossroads, the independent super PAC that backs GOP causes and candidates. Crossroads GPS is currently spending around $25 million to run ads critical of the president over a four week period in crucial battleground states.

The Democrats currently have a 53-47 (51 Democrats and two independents who caucus with the party) majority in the Senate, but they are defending 23 of the 33 seats up for grabs this November.