Senate Majority PAC, an outside group that supports Senate Democrats, is preparing a $3 million ad campaign against Charles G. Koch and David H. Koch, the libertarian-minded billionaire brothers who support conservative causes and have already poured millions of dollars into the 2014 midterm elections.

The Senate Majority PAC effort will last for roughly two weeks and span five states — Arkansas, Colorado, Louisiana, Michigan, and North Carolina. In an attempt to combat the Koch brothers, the $3 million buy will nearly double what Senate Majority PAC has spent on all television ads so far this cycle, around $3.6 million.

The group is spending about $500,000 in each of the states except North Carolina, where it is spending $1 million. Senator Kay Hagan of North Carolina is considered among the most vulnerable Democrats this election.