Wealthy political donors pumped millions of dollars into Democratic and Republican super PACs in the first quarter of the year — a sign that independent political groups will once again have a large impact on this year’s midterm contests.

One of the biggest beneficiaries of the amped-up giving was Senate Majority PAC, the super PAC providing air cover for vulnerable Senate Democrats. It pocketed $11 million for the quarter, with seven-figure donations from Chicago media mogul Fred Eychaner and New York hedge fund manager James Simons, according to campaign finance reports filed Tuesday.

The group, which has raised $19.5 million for 2014 campaigns, has spent $11 million.

Its fellow Democratic super PAC, House Majority PAC, raised more than $5.2 million for the quarter, fueled by a slew of six-figure contributions from labor unions and donors such as hedge fund manager Donald Sussman, Slim-Fast creator Daniel Abraham and author Amy Goldman Fowler. The group headed into April with nearly $7 million on hand.

The hauls suggest that the strategy of Democratic leaders such as Senate Majority Leader Harry M. Reid (Nev.) to repeatedly invoke the financial might of conservative donors Charles and David Koch has spurred giving on the left.

But Democrats still face a difficult task in mustering the kind of resources the Kochs and other conservative donors can pour into independent groups.

Americans for Prosperity, a nonprofit advocacy group, has already spent more than $34 million on ads against vulnerable congressional Democrats since October, according to a person familiar with the total. As a tax-exempt group, AFP is not required to disclose its fundraising, but the organization is expected to play a major role in key races throughout the year.

And Freedom Partners, another tax-exempt group that served as the funding arm of the Koch-backed political network in 2012, is spending directly on television ads this year. The group, which had already jumped into Senate races in Iowa and Colorado, has bought more than $250,000 in television spots set to run over the next two weeks in Louisiana. The ads will run in the Baton Rouge, Alexandria, Lake Charles, Lafayette and New Orleans markets, according to public data filed with television stations that will carry the advertising.

“While the Koch brothers are going to spend tens of millions of dollars trying to buy the U.S. Senate to advance their irresponsible and reckless agenda, we are going to continue to run smart, efficient and effective campaigns to stand up for seniors and hard-working families across the country,” said Ty Matsdorf, a spokesman for Senate Majority PAC.

Levi Russell, a spokesman for AFP, said that the Democratic super PAC has been running “demonstrably false and bizarrely disjointed attack ads” about the Kochs rather than addressing how President Obama and Senate Democrats have handled Obamacare.

“Now finally they’ve had a good fundraising month — good for them,” he said. “Hopefully now the PAC can hire a couple researchers to help get their facts straightened out.”

The intense fight for control of the Senate appears to have also jump-started giving by Republican donors who had been sitting on the sidelines. The super PAC American Crossroads raised almost $5.5 million in the first quarter, the group said Tuesday — nearly all of that money in the past month. That marks a dramatic reversal of fortune for the group, co-founded by strategist Karl Rove, which raised less than $4 million in all of 2013.

American Crossroads, which had more than $6.3 million at the end of March, recently began running ads on behalf of Republicans challenging Sens. Mark Begich of Alaska and Kay Hagan of North Carolina.

The group has not disclosed the names of the donors who refilled its coffers. The donors will be revealed when group files a fundraising report Sunday.

But Steven Law, president of the super PAC, said there has “been a noticeable rise in enthusiasm among our donors, in part due to the impressive slate of candidates who are strengthening the opportunity to win a Republican majority in the Senate.”

“These numbers put us in a solid position to continue impacting key Senate races where we can help elect Republicans who will clean up the mess that President Obama and Harry Reid are making in Washington,” he said in a statement.

There are signs, however, that GOP donors appeared less interested in giving money to outside groups backing House lawmakers, apparently confident that Republicans will keep their majority. The Congressional Leadership Fund, a super PAC that supports House Republicans, raised just $236,000 in the first quarter and spent nearly all of it.

The group’s sister nonprofit, the American Action Network, appears to be in better financial shape. The organization has announced spending amounting to $1.5 million in the past quarter, including $500,000 to help David Jolly win a special election for a Florida congressional seat.

Reid Wilson contributed to this report.