One of the main Democratic super-PACs engaging in House races raised $7.5 million in 2013.

That’s more than double the sum House Majority PAC raised in the off-year of the last election cycle, according to spokesman Andy Stone.

Stone said the group is planning to do battle with the Koch brothers and defend Democrats against GOP attacks, as well as go on offense against vulnerable Republicans.

“House Majority PAC is committed to combatting the efforts of the Koch Brothers and their right-wing allies, who are already stepping up to spend unprecedented sums in House races to promote their reckless agenda,” Stone said.

“Fortunately, as they witness a House GOP seemingly allergic to creating jobs or getting anything done, Democrats are both responding in a big way and recognize the continuing need to be aggressive," he added. 

One group affiliated with the Kochs, Americans for Prosperity, has already spent $16 million on attacks against Democratic lawmakers, and House Majority PAC is working to counter some of that offense. The Democratic group just announced plans to launch advertising in Arizona, where Democratic Reps. Ron BarberRon BarberGiffords to lawmakers avoiding town halls: 'Have some courage' Ten House seats Dems hope Trump will tilt House conducts moment of silence for Tucson shooting anniversary MORE and Ann KirkpatrickAnn KirkpatrickWomen make little gains in new Congress McCain wins sixth Senate term In Arizona, history and voter registration data gives GOP edge MORE have come under fire from Americans for Prosperity.

House Majority PAC has also run advertising defending vulnerable Democratic Rep. Nick RahallNick RahallWest Virginia is no longer Clinton country Solution needed: Rail congestion is stifling economic growth Lobbying World MORE (W.Va.) and attacking Republican Reps. Steve Southerland (Fla.), Rodney Davis (Ill.), Mike Coffman (Colo.), Joe Heck (Nev.) and John Kline (Minn.).