The conservative interest groups have spent more than $20 million in 2012 on the races at a time when Democrats are striving to retain their narrow majority in the Senate and regain control of the House, The Washington Post reported.
The newspaper said super PACs and non-profit groups, which have played a major role in the presidential race, also will be be a big factor in House and Senate campaigns.
Some Democratic lawmakers, including Senate Majority Leader Harry M. Reid of Nevada and House Minority Leader Nancy Pelosi of California, have increased fundraising efforts in recent months for liberal-leaning super PACs, which can raise unlimited amounts. But these efforts have drawn far less than their conservative opponents.
"The very reason our group was founded in the first place was to confront the flood of outside spending from the other side," said Andrew Stone, a spokesman for House Majority PAC, a Democratic group that focuses on House races.
"Our donors understand the dynamics at play here. The pitch is, we saw what happened in 2010 in so many congressional districts where the Democrats were just totally overwhelmed."
In this election cycle, interest groups on both sides have spent $29.8 million on congressional races, a Post analysis of Federal Election Commission reports found. That's more than twice the amount similar groups had raised at this point before the 2008 elections.