Democrats are embracing super PACs—the independent political money groups they once derided—and are easily outpacing Republicans in the race for cash, according to the most recent campaign finance filings.

In 2013, the three highest-profile Democratic super PACs focused on congressional elections collectively raised more than $22 million—about four times more than their five mainstream GOP counterparts, according to a Center for Public Integrity analysis of new filings submitted to the Federal Election Commission. Republicans, meanwhile, are again facing an intraparty struggle as Tea Party–affiliated super PACs take aim at mainstream Republicans in primary battles.

Super PACs, which roared to life after the U.S. Supreme Court’s 2010 Citizens United v. Federal Election Commission decision, differ from campaign committees and traditional political action committees because they can raise unlimited amounts of money from individuals, unions, and corporations. Liberals blasted the Citizens United decision and have scoffed at the groups it spawned. Still, super PACs are key to the Democrats’ plans for this year’s congressional elections.

Senate in play

The Senate Majority PAC, which aims to help Democrats retain control of Congress’ upper chamber, raised $8.6 million in 2013, and the House Majority PAC raised $7.8 million.

Additionally, the Democratic-aligned American Bridge 21^st Century super PAC—which specializes in opposition research, the type of information that often makes it into campaign ads as segments of damning video or audio—raised $5.9 million.

“Senate Majority PAC is necessary because we have to fight back against the Koch brothers and other conservative outside groups who are flooding millions of dollars into races trying to buy the Senate,” said spokesman Ty Matsdorf. “We can't fight with one arm tied behind our back.”

Billionaire brothers Charles and David Koch have spent millions backing conservative causes—the specter of their involvement this year has become an effective fundraising tool for the Democrats.

Former New York City Mayor Michael Bloomberg, an independent, ranked as Senate Majority PAC’s biggest backer last year, giving $2.5 million, while hedge fund executive Donald Sussman—the husband of Maine Rep. Chellie Pingree—was top donor to the House Majority PAC, at $850,000. Liberal billionaire George Soros, meanwhile, contributed $500,000 to American Bridge in December, earning him the top spot among that group’s 2013 donors. Several labor unions also pumped sizeable sums into the three groups’ coffers.

While Democratic-aligned groups are winning the fundraising race as of now, deep-pocketed donors can shift the scales overnight should they choose to pour new money into either super PACs or politically active nonprofits, the favored vehicles of the Kochs.

Poor showing for GOP

American Crossroads and the Conservative Victory Project, two super PACs connected to GOP strategist Karl Rove, each reported comparatively paltry receipts in 2013. American Crossroads, which was the top-spending super PAC during the 2010 midterm election, raised about $3.6 million in 2013. And the Conservative Victory Project raised just $16,500.

Two other GOP-aligned super PACs focused on helping the party retain control of the House—the Congressional Leadership Fund and the YG Action Fund—raised $1.1 million and $344,000, respectively. And a recently launched super PAC called America Rising, which specializes in opposition research, much like the Democrats’ American Bridge, raised about $478,000. One of its top donors was Restore Our Future, the super PAC that attempted to boost Republican Mitt Romney during the 2012 presidential race, that gave it $100,000.

Texas billionaire Harold Simmons, who died in December, ranked as the top giver to the Congressional Leadership Fund, at $200,000. His company, Contran Corp., ranked as the No. 1 donor to American Crossroads last year, having contributed $1 million.

American Crossroads spokesman Jonathan Collegio vowed that his group would “make a big impact in the 2014 elections” and help “win a GOP majority in the Senate and help expand the majority in the House.”

First, though, the GOP establishment must survive an intraparty fight, where conservative hardliners—armed with their own super PACs and nonprofits—are pushing Republican candidates rightward and even angling for their preferred candidates to triumph over incumbents they dislike.

Groups such as the Club for Growth and Senate Conservatives Fund—which have tapped into the energy and pocketbooks of Tea Party and anti-tax activists—are looking to replace GOP senators who they deem insufficiently conservative.

Republicans vs. Republicans

The potential targets include Mississippi Sen. Thad Cochran, Sen. Lindsey Graham of South Carolina, and Senate Minority Leader Mitch McConnell of Kentucky.

Club for Growth President Chris Chocola, a former Republican congressman from Indiana, has made it clear that his group is not afraid to go after incumbents. Club for Growth’s political machine, he said last year, “helps elect candidates who support limited government and free markets. Unfortunately, the two goals coincide less often than the Republican establishment cares to admit.”

The sentiment has also been echoed by Jim DeMint, the former South Carolina Republican senator who founded the Senate Conservatives Fund and now serves as president of the Heritage Foundation. “We must remember that there is a distinction between the Republican Party and the conservative movement,” DeMint declared at the 2013 Conservative Political Action Conference in Washington, D.C. “National Republican leaders have not advanced a conservative agenda for almost 20 years.”

Already, the Club for Growth’s super PAC has spent more than $200,000 on ads backing Mississippi state Sen. Chris McDaniel, who is challenging Cochran. And the Senate Conservatives Fund, through its super PAC and traditional political action committee arm, has spent nearly $1 million combined on ads backing either McDaniel or Matt Bevin, the conservative businessman challenging McConnell.

In 2013, the Club for Growth Action super PAC raised $2.6 million, and the Senate Conservatives Action super PAC raised $1.6 million. Wealthy home builder Bob Perry, a Texan who died in April, ranked as the top donor to Senate Conservatives Action, giving $1 million. Meanwhile, investor Virginia James of New Jersey and Illinois businessman Richard Uihlein each gave $500,000 to the Club for Growth’s super PAC last year, ranking as top donors to the anti-tax group.

Super PACs to “scare off opponents”

Super PACs supporting the candidacy of a single politician have become increasingly commonplace.

“Candidate-specific super PACs can help scare off opponents or signal to opponents and potential opponents that their favored candidate will have ample funding,” said Jessica Levinson, a professor at the Loyola Law School in Los Angeles.

That phenomenon is already unfolding in Kentucky, Mississippi, and South Carolina—and a host of other states from Alaska to North Carolina.

Last April, McConnell’s allies launched a super PAC called Kentuckians for Strong Leadership to aid in his re-election. The group raised $2.4 million in 2013. It received large contributions from the likes of real estate mogul Donald Trump ($50,000), coal executive Joe Craft ($100,000), and Florida-based NextEra Energy, a Fortune 200 utilities company ($100,000).

Graham’s supporters, likewise, created a group called the West Main Street Values PAC, which raised about $130,000 in 2013, including $50,000 from GOP fundraiser and former U.S. ambassador to Belgium Sam Fox and $25,000 from the political action committee of Boeing. And in mid-January, Cochran’s backers launched a super PAC called Mississippi Conservatives. That group has not yet disclosed any of its funders, but it has already spent more than $200,000 on pro-Cochran ads.

Republicans are not the only ones getting in on the action. Allies of embattled Democratic Sen. Kay Hagan of North Carolina recently launched a group called Wolfheel PAC, and Sen. Mark Begich of Alaska is being backed by a super PAC called Put Alaska First. The pro-Hagan Wolfheel PAC, which registered with the FEC in early January, has not yet been required to report its funders. Put Alaska First, meanwhile, collected $287,500 last year.

One of the largest donors to the pro-Begich political group was actually the Senate Majority PAC, which transferred $170,000 to Put Alaska First last year. A Washington-based fishing company headed by a Begich donor named Hae Joo “Helena” Park also contributed $100,000.

Then there’s We Are Kentucky, a super PAC that is backing Democrat Alison Lundergan Grimes, who is vying for McConnell’s Senate seat. It raised $260,500 since its formation in July. Two labor unions—the United Autoworkers and the plumbers and pipefitters’ United Association—each pitched in $100,000.

Ready for Hillary to play in midterms

At least two Democratic-aligned, candidate-specific super PACs are also already making preparations for the next presidential race. The first is Ready for Hillary, which was formed a year ago. The other is Priorities USA Action, which helped President Barack Obama win a second term in the White House and now plans to aid Hillary Clinton, should she opt to run in 2016.

Ready for Hillary spokesman Seth Bringman told the Center for Public Integrity that his group, which raised $4 million in 2013, will engage in races in which Clinton herself has endorsed candidates. “We will conduct additional efforts to amplify Hillary’s endorsements,” Bringman said. “Success in the midterm elections is vital to our party.”

Peter Kauffmann, a spokesman for Priorities USA Action, meanwhile, said that his group, which still has $3 million in reserves, does not intend to be involved in the 2014 midterm elections.

As Democratic and Republican groups alike look to keep increasing the size of their war chests, one thing’s for certain: The proliferation of super PACs will lead to a barrage of ads this year—particularly in the Senate’s battleground states.

“Residents of states with high-profile Senate races will be inundated with ads,” said Erika Franklin Fowler, a director of the Wesleyan Media Project, which monitors political advertising.

In many places, she continued, “ads have already started.”

Ben Wieder and Erin Quinn contributed to this report.

The Center for Public Integrity is a nonprofit, independent investigative news outlet. For more of its stories on this topic go to publicintegrity.org.