Conservative interest groups have dumped well over $20 million into congressional races so far this year, outspending their liberal opponents 4 to 1 and setting off a growing panic among Democrats struggling to regain the House and hold on to their slim majority in the Senate.

The surge suggests that big-spending super PACs and nonprofit groups, which have become dominant players in the presidential race, will also play a pivotal role in House and Senate contests that will determine the balance of power in Washington in 2013.

The money could be particularly crucial in races below the national radar that can be easily influenced by infusions of outside spending. One example came this week in Nebraska, where a dark-horse Republican Senate candidate upset two better-funded rivals in the GOP primary thanks in part to a last-minute, $250,000 ad buy by a billionaire-backed super PAC.

And in Indiana this month, veteran Sen. Richard G. Lugar was ousted in the GOP primary by challenger Richard Mourdock with the help of millions of dollars in spending by conservative groups. The Club for Growth, which backed a losing candidate in Nebraska, spent more than $2 million to help Mourdock in Indiana.

“We’re just getting started,” said Club for Growth spokesman Barney Keller, who said the group will soon begin training its fire on Democrats. “Our group has already had an impact on what the composition of Congress is going to look like next year. That’s our whole goal is to have an impact, to improve the gene pool in Congress.”

While Democrats welcomed the unexpected chance to compete for the Indiana seat, many are increasingly worried about the threat posed in the fall by well-funded conservative groups.

Leading Democratic lawmakers, including Senate Majority Leader Harry M. Reid (Nev.) and House Minority Leader Nancy Pelosi (Calif.), have stepped up their fundraising efforts in recent months on behalf of liberal-leaning super PACs, which can raise unlimited money but have fallen far short of matching 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 focused on House contests. “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.”

Interest groups on both sides have reported spending $29.7 million on congressional races so far this election cycle, according to a Washington Post analysis of Federal Election Commission reports.

The spending is more than twice the amount similar groups had spent at this point ahead of the 2008 elections. It’s also higher than the $25.2 million spent during the 2010 midterms, when several high-profile special elections and primary fights drove outside expenditures to new heights.

Spending among the largest groups favors Republicans by about 4 to 1, although that is due in part to a number of fierce Republican primary fights, the data show. GOP Senate primaries in Indiana and Texas, for example, have each drawn more than $4 million in spending by independent groups.

Conservative-leaning groups have also spent millions targeting Sens. Sherrod Brown (D-Ohio) and Claire McCaskill (D-Mo.) far ahead of the general election, ad data show.

In the House, the seat held by retiring Rep. Geoff Davis (R-Ky.) has become the biggest target for outside spending. The attention comes primarily from one man — a self-described self-employed student from Austin named John Ramsey — who has put $561,000 behind farmer and engineer Thomas Massie in the Republican primary.

A number of super PACs have become de facto arms of the two political parties, whose leaders can help raise money for the groups within legal limits of $5,000 per individual. But once introduced, those same PACs can hit up donors for unlimited amounts on their own and then use those funds to help the party candidates.

So far, the fundraising has primarily benefited conservative organizations such as the Club for Growth, the Congressional Leadership Fund and American Crossroads, the group connected to George W. Bush’s former White House adviser Karl Rove, which plans to spend up to $100 million on House and Senate races through its super PAC and nonprofit arms.

Overall, congressionally focused super PACs have raised $100 million so far this election cycle, including $28 million by American Crossroads, according to FEC data.

Democratic-leaning groups have lagged badly by comparison. House Majority PAC has reported raising $4.5 million since last year, primarily from unions, and had $1.7 million on hand at the end of March.

The group’s Republican counterpart, the Congressional Leadership Fund, reports having three times as much cash on hand — nearly all of it from a pair of $2.5 million donations from casino magnate Sheldon Adelson and his wife.

“Groups like ours are a counterbalance to the huge efforts being spent on the left by unions, environmentalists and trial lawyers who have been bankrolling Democrats for ages,” said Dan Conston, a spokesman for the Congressional Leadership Fund and its parent organization, the nonprofit American Action Network.

Democratic operatives say they hope to narrow the fundraising gap in the months ahead and note that many conservative groups have depleted their coffers waging primary fights or spending money in states that are favorable to Democrats.

Liberal groups have been hobbled by widespread reluctance among Democrats to support unlimited-spending groups, which have been criticized by President Obama and others as unhealthy for democracy. But Democratic strategists say donors are coming around.

“It’s no secret that it’s not the way we would prefer to do things,” said J.B. Poersch, a strategist for Majority PAC, which is focused on bolstering Senate Democrats. “But we’re also recognizing that we have no choice but to compete. . . . You can’t just give away the store.”

At least one candidate, McCaskill in Missouri, is focusing much of her reelection campaign on the influence of outside conservative groups, which spent as much as $4.5 million attacking her in the early months of the race, according to Democratic tracking figures. In an appearance this month on “The Colbert Report,” McCaskill complained about the impact of “sleazy, slimy secret money” on the race.

“Voters are very attuned to the fact that these right-wing outside groups are spending big and they expect something in return for their investment,” said Matt Canter, a spokesman for the Democratic Senatorial Campaign Committee. “Voters aren’t taking too kindly to that proposition.”

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