WASHINGTON -- As the U.S. heads into the third national election since the Supreme Court's Citizens United ruling, super PACs are still raking in the largest share of their contributions from those giving $500,000 or more. What's different this go-around is the sharp drop in the number of Republicans on the list of mega-donors.

Overall, the mega-donors, who gave $106 million through March 31, accounted for 61 percent of all contributions to super PACs this election cycle, according to Federal Election Commission filings analyzed by The Huffington Post.

But of these 62 donors, 42 gave to Democratic-aligned groups while just 18 gave to Republican-aligned groups. Another two donors contributed to nonpartisan super PACs.

The relative dearth of Republican mega-donors underscores the extent to which major conservative contributors have fled the realm of super PACs, whose donors must be disclosed, for the "dark money" world of political nonprofits.

Source: Federal Election Commission.

Dark money nonprofits, which are not required to name their donors, have dominated spending in this electoral cycle. On the right, those connected to the billionaire Koch brothers have already dropped nearly $40 million on issue-based ads in contested House and Senate races. The U.S. Chamber of Commerce and the American Action Network have combined with Republican-aligned groups to spend $7 million on election ads.

Democratic-aligned dark money nonprofits -- like the League of Conservation Voters, Patriot Majority USA and other environmental groups -- have also spent millions, although far less than their conservative kin.

In the last two elections, Republican-allied donors represented 63 percent of all contributions to super PACs, which can take in unlimited amounts of money. In this election, however, Democratic groups have made up 55 percent of all super PAC spending so far, according to the Center for Responsive Politics.

Despite concerns that liberal contributors were more focused on a potential 2016 presidential bid by Hillary Clinton, individual Democratic mega-donors made large contributions in the first three months of 2014. The biggest of these this year came from media mogul and LGBT activist Fred Eychaner, who gave $4 million to Senate Majority PAC and $500,000 to House Majority PAC, both of which aim to elect Democratic candidates.

Other major individual donors to Democratic groups in the first quarter of the year included Stryker Corporation heir and LGBT activist Jon Stryker, who gave $700,000 to House Majority PAC and $250,000 to Senate Majority PAC; investor George Soros, who gave $500,000 to American Bridge 21st Century; and health care tech innovator Paul Egerman, who gave $350,000 to American Bridge.

Labor unions have given $27.8 million in the 2014 cycle, with $5.9 million coming in the first three months of this year. Nearly all of the union contributions went to super PACs supporting Democrats. Defending Main Street SuperPAC, a group that backs moderate Republicans, raised $750,000 from labor unions.

Unlike Democratic mega-donors, many wealthy conservatives are funding super PACs involved in intra-party primary fights -- including the Club for Growth and Senate Conservatives Fund. Richard Uihlein, Virginia James, Robert Mercer, Robert Arnott and Sean Fieler have all targeted donations to groups fighting the GOP establishment or moderate social and economic policies.

But American Crossroads, the main super PAC of the Republican Party establishment, has also attracted its share of large contributions. After struggling with fundraising last year -- a difficulty widely blamed on its poor performance in the 2012 elections -- the group co-founded by Karl Rove raised more than $5 million in the first three months of 2014. Previous donors, including Univision founder Jerrold Perenchio, Public Storage founder B. Wayne Hughes and coal titan Joseph Craft, stepped up.

Republicans can no longer rely on two of the party's most prolific donors of the last two decades. Though they made the current mega-donor list for contributions given last year, Texans Harold Simmons and Bob Perry both died in 2013.

New donors are stepping up to take their influential place. Hedge fund executive Paul Singer and investor Seth Klarman have targeted millions in contributions to support Iran war hawks and pro-gay marriage Republican candidates. They are already spending big money through Ending Spending Action Fund -- a super PAC founded by another mega-donor, Joe Ricketts -- in an effort to defeat Rep. Walter Jones (N.C.), the only anti-war and anti-Wall Street Republican in Congress.