The first generation of super PACs operated as shadow party committees, embodied in American Crossroads, the pro-Republican behemoth co-founded in 2010 by Karl Rove and Ed Gillespie.
Next came candidate-specific super PACs, like those that last year aided President Barack Obama and his GOP rival, Mitt Romney.
The latest iteration is the single-issue vanity super PAC — a group backed by a single, wealthy donor focusing on an issue of national importance, such as climate change or gun violence.
Case in point: Thomas Steyer.
An environmental activist and former hedge fund executive, Steyer invested more than $11.1 million into his two super PACs last year. That’s more than any other individual, union or company gave to super PACs in 2013, according to a Center for Public Integrity review of campaign finance records.
Steyer’s NextGen Climate Action Committee and CE Action Committee hammered home the message that politicians need to take action on global warming during the Virginia gubernatorial race and the special U.S. Senate election in Massachusetts.
This stands in stark contrast to the 2012 election cycle when Steyer did not donate a dime to super PACs.
Steyer "is committed to engaging in campaigns where climate is on the ballot," said Suzanne Henkels, the deputy press secretary for NextGen Climate Action, who added that the CE Action Committee "is in the process of winding down its operations."
Super PACs became a household term following the U.S. Supreme Court’s Citizens United v. Federal Election Commission decision in 2010. Unlike candidates, these political committees can accept unlimited donations from billionaires, corporations and unions and use the money to pay for ads that support or oppose candidates.
Republican groups, like American Crossroads, were quicker to welcome the new rules than Democrats. This election cycle, however, Democrats have embraced the big-money groups in a bid to retain control of the U.S. Senate.
No. 2 on the list of 2013’s super donors is a billionaire who was once a member of each party and is now an independent funding his own issue-driven super PAC.
Former New York City Mayor Michael Bloomberg contributed $8.7 million to super PACs last year, mostly to his own Independence USA PAC, a group that favors stricter gun control laws.
The outfit backed Democrat Terry McAuliffe in his winning 2013 bid to be Virginia's governor. It was also active in a U.S. House race in Illinois and the special U.S. Senate election in New Jersey.
Bloomberg has said the super PAC is designed to support moderates who “will help protect Americans from the scourge of gun violence, improve our schools and advance our freedoms," including gay couples’ right to marry.
Bloomberg gave about $6 million to the Independence USA PAC in 2013.
He also contributed $2.5 million to the pro-Democratic Senate Majority PAC and $250,000 to Americans for Responsible Solutions, a pro-gun control super PAC founded by former Democratic Rep. Gabby Giffords of Arizona.
Then there’s Jonathan Soros, the son of financier George Soros.
The younger Soros contributed more than $1 million last year to a hybrid super PAC he co-founded, known as Friends of Democracy. That was enough to rank Soros among the largest Democratic-leaning super PAC donors in 2013, although not quite enough to earn him a spot on the top 10 list.
The elder Soros, for his part, contributed about half as much as his son last year, including $500,000 to the Democratic-aligned American Bridge 21st Century super PAC, $25,000 to the Ready for Hillary super PAC and $5,000 to Friends of Democracy.
Friends of Democracy is designed to be the anti-super PAC super PAC. It supports candidates who favor campaign finance reform and seeks to “stop the power of money in politics.” As a hybrid super PAC, it can make direct donations to candidates from one fund that accepts only contributions of limited size and produce political ads from another fund that is unrestricted.
Numerous labor unions peppered the list of the 10 largest super PAC donors in 2013, including the National Education Association (No. 4), the United Brotherhood of Carpenters and Joiners (No. 5), the AFL-CIO (No. 7) and AFSCME (No. 8).
Also on the list: the Democratic Governors Association (No. 3), which largely used its super PAC to funnel money to McAuliffe's campaign in Virginia, and Texas trial lawyers Steve and Amber Mostyn, who mostly gave through their law firm (No. 10).
In all, the top 10 super donors collectively gave more than $47 million, accounting for nearly one-third of the roughly $150 million raised by super PACs last year, according to the Center for Public Integrity’s research.
The biggest Republican super PAC donor in 2013 was Bob Perry, who contributed $3.1 million and whose death in April has left a void in GOP fundraising circles.
That sum ranked him as No. 6 on the super donor list.
The largest beneficiary of Perry’s giving was a hybrid super PAC called Texans for a Conservative Majority, which received $2 million. That group has actively worked to help Sen. John Cornyn, R-Texas, fend off a conservative primary challenger.
Many of the deep-pocketed GOP donors who ranked among the most generous givers during the 2012 election cycle have yet to open their checkbooks for super PACs during this election cycle.
Casino magnate Sheldon Adelson did not contribute a cent to super PACs in 2013. Nor did his wife Miriam Adelson. The couple contributed more than $90 million to such groups during the previous election cycle.
Similarly quiet were GOP mega-donors Joe Ricketts, Robert Rowling and William Koch, the lesser-known brother of billionaire industrialists David and Charles Koch. The more famous Koch brothers have typically used nonprofits, rather than super PACs, to weigh in on policy debates and elections.
The only other Republican to break the list of the top 10 super PAC donors in 2013 was John Jordan, a California vintner who ranked ninth overall after contributing $1.7 million to a super PAC that backed underdog GOP Senate candidate Gabriel Gomez in the Massachusetts special election last June.
His super PAC has since disbanded.
Ben Wieder contributed to this report.