Volunteers help roll up a giant banner printed with the Preamble to the United States Constitution during a demonstration against the Supreme Court's Citizens United ruling at the Lincoln Memorial on the National Mall October 20, 2010 in Washington, DC. Chip Somodevilla | Getty Images

In 2010, the Supreme Court handed down a ruling that effectively flooded political campaigns with limitless money from corporations and wealthy donors. That decision came in a case called Citizens United vs. the Federal Election Commission. The top court ruled that corporations, unions and other organizations had a First Amendment right to spend what they like to influence federal elections. The 5-4 decision was based in part on the idea that disclosure of these investments would act as a natural check against improper activities. But the ruling triggered a wave of political action committees, super PACs and so-called "dark-money" groups, which don't have to reveal their donors. During the 2008 presidential campaign, which pitted Democrat and eventual winner Barack Obama against Republican Sen. John McCain, records show that dark-money groups only put up $25 million toward that contest, according to the nonpartisan Center for Responsive Politics. Four years later, following the Citizens United decision, such groups spent seven times that total. After nearly 10 years and hundreds of millions of dollars spent, the groups unshackled by Citizens United are facing more opposition than ever. Even its original supporters have acknowledged unexpected outcomes. Justice Anthony Kennedy, who has since retired, said in a 2015 interview at Harvard Law School that disclosure is "not working the way it should." That was five years after he had argued that "the advent of the Internet" and "prompt disclosure of expenditures" would "provide shareholders and citizens with the information needed to hold corporations and elected officials accountable for their positions and supporters." The power and influence of PACs and super PACs have become a key point in the 2020 presidential campaign, particularly as two of the leading candidates, Sens. Elizabeth Warren and Bernie Sanders, have refused to accept money from super PACs. Instead, they've focused on funding their campaigns mainly through small-dollar donations, also known as grassroots funding.

Democratic presidential candidate Sen. Bernie Sanders (I-VT) (R) and Sen. Elizabeth Warren (D-MA) greet each other at the start of the Democratic Presidential Debate at the Fox Theatre July 30, 2019 in Detroit, Michigan. Justin Sullivan | Getty Images News | Getty Images

"The next president of the United States can bring this era of dark money to an end — and Democrats need to link arms and show that we can win by pledging to run a grassroots campaign," Warren told CNBC in a statement. "To make our government work for everyone, not just those at the top, we need to beat Donald Trump in 2020 — but that alone is not enough. To make real change come January 2021, we need to attack corruption head-on, and that starts with a constitutional amendment overturning Citizens United." The fight over campaign finance has become an all-out war, without an end in sight, but it's been brewing for years. CNBC interviewed donors, strategists and legal experts, and reviewed data compiled by the Center for Responsive Politics, to explain how we got here, and what might come next.

Mitch vs. the Maverick

One of the key fights that preceded Citizens United actually pitted two top Republicans against each other: the late McCain of Arizona and Sen. Mitch McConnell of Kentucky. McCain became known as a maverick for his tendency to buck the party line on certain issues, including campaign finance restrictions. This was encapsulated by the bill he co-authored with then-Sen. Russ Feingold, a Democrat from Wisconsin. Known as McCain-Feingold, the law went into effect in 2002 when President George W. Bush, himself a Republican, signed the bill. McConnell, who would eventually become the Senate minority leader and then majority leader, believed the measure was an infringement on the First Amendment.

Sen. John McCain, R-Ariz., and Senate Minority Leader Mitch McConnell, R-Ky., during a news conference on health care overhaul. Scott J. Ferrell | Congressional Quarterly | Getty Images

"He was the number one adversary to John McCain on this issue, and they were fierce competitors for many years," former McConnell chief of staff Josh Holmes told CNBC. "There was no element of it that they agreed on," he added. The law limited the amount of money that could be given and, in some cases, used, by corporations, labor unions and wealthy individuals during an election. Citizens United, however, curtailed aspects of the bill, including a provision that limited corporate-paid "electioneering communications" that named a candidate within 30 days of a primary or caucus or 60 days of a general election. It did not, however, do away with McCain-Feingold's ban on foreign nationals contributing to campaigns. McCain, who ran for president in part on reforming campaign finance laws, said in a PBS interview in June 2012 that the Citizens United ruling was the "most misguided, naive, uninformed, egregious decision by the United States Supreme Court in the 21st century." He offered a dire prediction of what would happen in 2012 and beyond. "There will be scandals. There's too much money washing around political campaigns today. It will take scandals and then maybe we can have the Supreme Court go back and revisit this issue," he said then.

The 2012 deluge

In the aftermath of Citizens United, allies of Obama and Republican candidate Mitt Romney were prepared to spend millions on the 2012 presidential election. The stakes were further heightened by a decision that Obama made four years earlier. His first presidential campaign saw such a deluge in micro-donations, he eschewed the federal funding for his campaign that his then-opponent McCain had committed to. After Obama's decision to reject federal funding, essentially all candidates have followed suit.

Republican presidential candidate Mitt Romney (L) and U.S. President Barack Obama talk over each other as they answer questions during a town hall style debate at Hofstra University October 16, 2012 in Hempstead, New York. John Moore | Getty Images

Romney, a former Bain Capital executive, needed an extra boost against an incumbent with a historically strong campaign war chest. Obama, on the other hand, was fighting a slight dip in the polls and the outrage from Republican circles on the passing of his signature legislative achievement, the Affordable Care Act, also known as Obamacare. Through the super PAC Restore our Future, Romney had the support of people who were close to him during his initial 2008 run for president. Longtime Romney associate Charles Spies, who was one of the architects of that committee, said in an email that the Citizens United ruling allowed donors to help keep Romney in the fight against Obama. "The CU (Citizens United) ruling sent a message to individual and corporate major donors that it was OK for them to participate in the political process again through independent expenditures," Spies told CNBC. "Democrats and the mainstream news media prefer to have the marketplace of ideas to themselves and so were apoplectic about the idea the conservatives could also get their message out through super PACs, and that played out with super PACs helping keep Mitt Romney afloat in 2012, despite his campaign getting dramatically outspent by the Obama campaign." Restore our Future ended up being the top super PAC during that election cycle, spending over $142 million, with $82 million spent to target Obama directly, according to the Center for Responsive Politics. The top donors by the time Romney fell to Obama were Las Vegas Sands CEO Sheldon Adelson and his wife, Miriam. They combined to give at least $30 million to the PAC. Adelson would eventually go on to be one of Trump's top financiers when he ran for president in 2016.

US business magnate Sheldon Adelson and his wife Miriam Ochsorn pose for photographers after attending the first presidential debate between US President Barack Obama and Republican challenger Mitt Romney in Denver,Colorado on October 3, 2012. Nicholas Kamm | Getty Images

On the other side was Priorities USA Action, which was founded in 2011 by Bill Burton, a former Obama deputy White House press secretary, and Sean Sweeney, a top aide to Obama's one-time chief of staff Rahm Emanuel. The PAC raised $79 million during Obama's reelection campaign and spent just over $65 million against Romney. Their top donors included Hollywood movie executive Jeffrey Katzenberg and billionaire hedge fund manager James Simons. Each gave the PAC at least $3 million. Burton and a spokesman for Priorities USA action did not return a request for comment.

Dark money creeps in

The dark-money organizations also saw an uptick in spending in the 2012 presidential election cycle following Citizens United. These groups are also known as 501(c)(4) nonprofits, or "social welfare groups," which the IRS allows to keep their donors anonymous. They often invest in elections when they want to back certain candidates who agree with their policy goals. While public companies have shareholders and budgets to answer to, these organizations don't have any such checks. "The Supreme Court held that disclosure was alive and well. However, through 501(c)4 organizations, we've not seen that kind of disclosure," said Kenneth Gross, who is a partner at Skadden, Arps, Slate, Meagher & Flom focusing on campaign finance law and used to be associate general counsel of the Federal Election Commission. In 2012, social welfare groups spent over $143 million, including $88 million against Democrats and $9 million against Republicans, records show. It was historically the most spent on an election by "dark-money" institutions. Voters saw a network that was created by two libertarian leaning billionaires, Charles and David Koch, take aim at Obama throughout his reelection.

Charles and David Koch Miller Hawkins | MSNBC

During one of their donor retreats in California in 2012, the Koch brothers, along with their allies, reportedly declared they would spend up to $100 million against Obama and would specifically target the health-care bill that passed just years earlier. They had been strong opponents of the president's proposal since he entered the White House in 2009. Their nonprofit, Americans for Prosperity, spent $36 million that cycle, including millions on anti-Obamacare ads. In 2016, Trump spent $66 million of his own money to campaign against Democrat Hillary Clinton. Trump and Clinton both raised hundreds of millions of dollars, with Clinton edging out the billionaire turned presidential candidate in small dollar funding. Each had outside groups backing them with millions spent on super PACs and dark-money groups. Investments by nonprofits were significantly less then in 2012, however, records show.

Grassroots, billionaires and Trump

While Citizens United has allowed big-money interests to saturate the political campaign market, it has also generated a backlash that has given rise to grassroots fundraising. "It [Citizens United] has made grassroots donations and small-dollar donations much more significant. The small-dollar donations and the grassroots army is the front line defense to super PACs," longtime Democratic donor Robert Zimmerman told CNBC. Several Democrats running for president in the 2020 primary have pushed the idea that primary contenders who are linked to wealthy donors and outside groups should not be considered viable candidates.

Sen. Elizabeth Warren (D-MA) gestures as democratic presidential candidates (L-R) South Bend, Indiana Mayor Pete Buttigieg, former Vice President Joe Biden, Sen. Bernie Sanders (I-VT), Sen. Amy Klobuchar (D-MN), and Tom Steyer await the start of the Democratic presidential primary debate at Loyola Marymount University on December 19, 2019 in Los Angeles, California. Mario Tama | Getty Images

Warren and Sanders have been the most outspoken against the influence of money in politics, and it has pressured several of their rivals to adopt similar stances. Mayor Pete Buttigieg, former Vice President Joe Biden, Sens. Amy Klobuchar and Cory Booker have said they will not accept corporate PAC money or contributions from lobbyists. Biden and Booker still have super PACs supporting their candidacies, despite the candidates' attempts to distance themselves from them. Trump's reelection campaign has also gone from being associated with a ton of outside PAC support in 2016 to relying on a standard committee, a separate joint fundraising operation with the Republican National Committee, and an external super PAC — America First Action. Trump's campaign has received $79 million in donations of under $200, accounting for almost 37% of their total funding.

President Donald Trump hosts a Merry Christmas Rally at the Kellogg Arena on December 18, 2019 in Battle Creek, Michigan. Scott Olson | Getty Images

Citizens United, however, has also led to another kind of potentially unintended consequence: the rise of billionaire candidates who can claim they are not beholden to anyone else's interests. "I think ironically the Citizens United decision didn't permit self-financing in a way that wasn't possible before, but what it did was create this accurate sense that campaigns were being dominated by corporate interests and wealthy donors which created an appetite for candidates who weren't reliant on that type of support," said Noah Bookbinder, executive director of the campaign finance watchdog, Citizens for Responsibility and Ethics. "One of the ways you don't have to be reliant on that type of support is to have the money yourself." Two billionaire Democrats are running for president. One of them, former New York Mayor Mike Bloomberg, has spent over $100 million on TV ads since he entered the primary just before Thanksgiving, according to data gathering firm, Advertising Analytics. Tweet But all the backlash and the presence of uber-wealthy candidates haven't scared away dark-money groups. They're still around. "They've just gotten better at hiding it from the Federal Election Commission," Bookbinder said. "The dark-money groups are now spending earlier in the election voting on more issue-related ads. It's just harder to trace."