A conservative group backed by billionaire activist Charles Koch is launching a new multimillion-dollar ad campaign across three states to boost GOP Senate candidates in tough midterm election fights.

Americans for Prosperity (AFP) is putting nearly $5 million behind ads set to go live on Thursday in Wisconsin, Missouri and Tennessee attacking the Democratic Senate candidates for their records on taxes, spending and health care.

Baldwin leads Republican Leah Vukmir by 10 points in the RealClearPolitics average, although Vukmir only officially became the GOP Senate candidate two weeks ago.