WASHINGTON — Senate Majority PAC, a group that supports Democratic Senate candidates, is preparing a $3 million advertising 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 group’s 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’ influence, the $3 million buy will nearly match what Senate Majority PAC has spent on all of its other television ads so far this cycle, around $3.6 million.

The group is spending about $500,000 in each of the five states except North Carolina, where it is spending $1 million to help Senator Kay Hagan, who is considered one of the most vulnerable Democrats up for re-election this year.

Americans for Prosperity, a Koch-backed group, has spent up to 10 times as much as the Senate Majority PAC, quickly becoming one of the biggest players in this year’s election. The group’s goal is not merely to influence particular issues or races but to also win voters over to their broad philosophy: that big-government solutions are risky and ineffective.