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In 2010, the largest reported individual contributors to federal campaigns in American politics were Robert and Doylene Perry, owners of Perry Homes, who donated about $7.5 million to support Republican and conservative candidates. In 2018, the largest reported contributors were casino magnate Sheldon Adelson and his wife, Miriam, who contributed about $122 million in outside money to support such candidates, representing a 16-fold increase over the Perrys’ 2010 contributions, according to data collected by the Center for Responsive Politics. What explains this dramatic shift in American elections, where the wealthiest Americans get to have even greater influence over who is elected and what policies elected officials pursue? The Supreme Court’s 2010 opinion, Citizens United v. Federal Election Commission.

In 2010, Citizens United held that corporations have a First Amendment right to spend sums independently to support or oppose candidates for office. Looking at the amount of direct corporate spending in elections over the past decade, one might think that Citizens United was a bust. Few for-profit corporations spend money in their own names boosting or dissing candidates. But this case helped to usher in a sea change in American elections, and its influence on the decade that followed is hard to overstate. We’ve seen an explosion of outside, often-undisclosed money in elections, candidates skirting campaign finance rules by having shadow “super PACs,” and dangerous foreign interference in our elections. And that pivotal opinion contains all the tools the Supreme Court needs to get rid of remaining campaign contribution limits.

The roots of Citizens United go back more than 40 years, to the court’s 1976 opinion in Buckley v. Valeo. In that case, the court held that limiting what individuals may spend on elections imposes steep free speech costs that could violate the First Amendment. Such limits could be justified under “strict scrutiny” only if the government could show a compelling interest in limiting this spending. The court held that preventing corruption was indeed compelling, but limits on independent spending could not be justified on anti-corruption grounds: If the spending was independent of the candidate, the court reasoned, it could not corrupt the candidate or create an appearance of corruption, even if the money was being spent in support of that candidate. The court also rejected an argument that limiting a billionaire’s influence in elections, for example, could be justified by a desire to level the electoral playing field, holding such an interest in political equality “wholly foreign to the First Amendment.”

Still, until 2010, courts continued to enforce long-standing limits on corporate spending. The Supreme Court held in 1990 that corporate limits could be justified by a compelling interest in preventing “the corrosive and distorting effects of immense aggregations of” corporate wealth on the political process. A 2003 case reaffirmed this principle and upheld new limits on corporate and labor union spending.

Citizens United abruptly reversed that trend by deciding that corporations are entitled to the same rights that Buckley recognized for people, with Justice Anthony Kennedy declaring for the majority that the sale of access to elected officials is not corruption and that “the appearance of influence or access, furthermore, will not cause the electorate to lose faith in our democracy.” The one bright spot in an otherwise bleak opinion was the court’s reaffirmation of the constitutionality of laws requiring those who contribute or spend money in elections to disclose their identities. The court agreed with the government that such laws are necessary for an informed electorate.

Few experts expected to see large business corporations spending directly in candidate elections after Citizens United. Thanks to disclosure rules, corporations typically would be skittish to back one candidate over another for fear of alienating customers who would learn of the spending from campaign advertising or from the press. But Citizens United has had profound effects on campaigns through less direct channels. Here are a few of the ways our elections have deteriorated since:

Super PACs and outside spending in campaign elections skyrocketed. This is a trend that began before Citizens United but has accelerated since. Just comparing presidential election years, we saw $338 million in outside spending in the pre­–Citizens United 2008 election, compared with more than $1 billion in 2012 and $1.4 billion in 2016. The 2018 elections featured the first midterm election cycle with more than $1 billion in outside spending.

After Citizens United, courts and the Federal Election Commission issued decisions that led to the creation of super PACs, political committees that spend money independent of candidates to promote or oppose candidates for office. They can take unlimited donations from individuals, corporations, and labor unions. The legal rationale flowed directly from Citizens United: If independent spending cannot corrupt, then contributions to fund independent spending cannot corrupt either. Thus corporations started giving to super PACs to gain influence without the corporations’ names being right on the face of campaign advertisements. The Center for Responsive Politics lists 1,185 corporations that gave money to super PACs in the 2018 election cycle. Koch Industries leads the list with a $2 million donation while others gave up to $1 million.

Corporations and others can also hide contributions to “social welfare” groups and other organizations that are not required to disclose their donors. These groups now flourish despite Citizens United’s endorsement of the value of campaign finance disclosure. And although billionaires and other very wealthy individuals have had the right since the 1976 Buckley case to spend large sums independently supporting or opposing candidates, many were skittish to do so until the rise of super PACs, which have made it easier to remain in the shadows. In the 2018 election season, it was not just the Adelsons with their $122 million contribution; former New York Mayor and current presidential hopeful Michael Bloomberg gave more than $95 million and fellow-billionaire candidate Tom Steyer and his spouse gave nearly $74 million, according to CRP data.

Super PACs became shadow campaigns. The whole conceit of Buckley and Citizens United is that independent spending cannot corrupt because there can be no coordination with candidates and therefore no opportunity for a quid pro quo. But in fact, thanks to porous FEC rules, candidates and supportive super PACs can legally do just about anything together, short of sitting down to plot a campaign advertising strategy. In a notorious case from 2016, a super PAC supporting Carly Fiorina essentially ran her campaign. It is common for these “independent” organizations to be run by relatives or former employees or strategists of candidates, rendering them reliable surrogates for the campaigns themselves.

Foreign money in elections grew. As I recently explained in Slate, shortly after the Supreme Court decided Citizens United v. Federal Election Commission in 2010, President Barack Obama warned in his State of the Union that the ruling could theoretically permit foreign nations, acting through corporations, to try to influence the outcome of U.S. elections. Justice Samuel Alito, in attendance, mouthed that this was “not true.”

Alito presumably protested because Citizens United did not answer the question whether a ban on independent foreign spending in elections violated the First Amendment the way a corporate spending ban would. But Obama turned out to be right: Foreign entities are exploiting the rise in outside groups created by Citizens United and porous campaign disclosure rules to get money and influence in our elections. Indeed, the recent indictment of Rudy Giuliani associates Lev Parnas and Igor Fruman reveals the funneling of money from a Ukrainian oligarch, through pass-throughs including an LLC, into a pro-Trump super PAC and even directly into campaigns. The Washington Post reported that Fruman and Parnas got a chance to tell Trump to his face that he should fire Marie Yovanovitch, the U.S. ambassador to Ukraine, thanks to a promised large contribution to the Trump super PAC.

And Trump has not been the only beneficiary of foreign money. A recent indictment charged a number of people including George Nader (a bit player in the Mueller investigation who is currently awaiting trial on child pornography charges) with funneling $3.5 million into American elections. Priorities USA, the Democratic super PAC, received $1 million of these funds. Foreign money likely continues to pour in as the 2020 election gets underway.

The seeds of further destruction were planted. Back in 1976, the Supreme Court upheld the constitutionality of campaign contribution limits, on grounds that they prevented corruption of candidates and the appearance of corruption. The 2010 Citizens United opinion claimed not to touch this aspect of Buckley. But when the court in Citizens United held that independent spending could not corrupt, it considerably narrowed the definition of “corruption” to make it harder to uphold campaign contribution limits on anti-corruption grounds.

The Supreme Court in 2014 struck down a federal limit in the McCutcheon v. Federal Election Commission case applying this stingier definition of corruption. In November of this year, the court sent an Alaska campaign contributions case back to the 9th U.S. Circuit Court of Appeals to take a more skeptical look at the constitutionality of the state’s $500 contribution limits.

What’s next? People are understandably focused on more pressing questions than money in politics right now, including impeachment, the security of our voting technology, and whether American democracy can survive the 2020 elections. But if one cares about free and fair elections, the status of our campaign finance system is deeply troubling. And thanks to Congress’ inaction on updating disclosure rules, much of this influence will happen in secret. The decade of Citizens United has been a bad one for democracy, but the next decade could be far worse as a new, more radical Supreme Court prepares to blow up the final limits on money going to candidates for election.

Read the rest of Slate’s coverage of the end of the decade.