It paved the way for the creation of so-called super PACs, and unlimited, undisclosed contributions to outside groups that are often impossible to track. As a consequence, political operatives expect the 2016 presidential contest to attract billions
from deep-pocketed donors and dark money groups.
The growth of money in politics has also caused calls for reform to grow louder, and on Wednesday proponents of campaign finance reform protested on Capitol Hill and across the nation. A rare protest
broke out in the Supreme Court to mark the fifth anniversary of Citizens United.
But even as progressives are trying to change the rules of the game, Democrats have been forced to play ball.
According to an analysis from the Brennan Center for Justice
, a nonpartisan government transparency group, outside spending on Senate races has more than doubled since 2010, to $486 million during the midterms.
And more of that money than ever before comes from a small pool of wealthy donors, according to the Center for Responsive Politics
, which found that just 1% of donors gave nearly 68% of the $828 million raised by super PACs in 2012 — a number that's only expected to grow in 2016.
To keep up with the influx of outside spending, in many cases Democrats have been forced to reverse stances in favor of tougher regulations and public financing and embrace the super PAC.
President Barack Obama has been perhaps the biggest disappointment for progressives on this front. He once promised to pursue reforms, but turned his campaign committee into an advocacy group to collect unlimited donations for his re-election campaign and has been largely quiet on the issue.
But he and other proponents of reform face a difficult choice: Speak out on the issue, with dark money assistance, and you're a hypocrite — or eschew the system that you're trying to change, and get left behind.
That may be why Obama made only passing mention of the issue during his State of the Union on Tuesday night, declaring that "a better politics is one where we spend less time drowning in dark money for ads that pull us into the gutter."
And on Wednesday, he merely issued a statement, saying the Citizens United decision was "wrong, and it has caused real harm to our democracy," but offering no proposals for reform.
Still, reformers say they're still hopeful that they can enact change, at least at the state level, citing the growing public opposition to money in politics.
David Donnelly, executive director of Every Voice, a campaign finance reform group, suggested the issue could become a defining one in the 2016 elections, with potential primary fights brewing on both the right and the left.
"This is an untenable situation out there. There's gotta be some candidate that will seize on this issue and run with it," he said.
And on Wednesday, two progressive senators named as potential primary challengers to former secretary of State Hillary Clinton — Bernie Sanders of Vermont and Elizabeth Warren of Massachusetts — did express their support for reforms.
But while Americans widely oppose the influence of money in politics, reformers haven't yet figured out how to make it a winning campaign issue.
Democrats fell short with their campaign to turn the billionaire Charles and David Koch brothers, who spent over $30 million on the 2014 midterms through a murky web of super PACs and nonprofits, into a liability for Republicans — and Republicans missed the mark in trying to do the same with billionaire environmentalist and Democratic donor Tom Steyer.
Still, Donnelly said, the fight for reform "is a marathon, not a sprint" — and he holds out hope Obama could still act on the issue.
"There's two years of possibilities ahead of us — the book has yet to be written on that," he said.