Campaign spending by outside groups, such as super PACs, has more than doubled in the past five years — and most of it is coming from the super-rich, according to a new analysis of spending on Senate races by the Brennan Center for Justice.

Spending by outside groups — those who operate independently of political candidates — increased in the wake of the 2010 Citizens United Supreme Court decision, which ruled that it was unconstitutional to restrict spending by corporations, associations and labor unions. The decision held that curbing these groups’ spending limits inhibited their freedom of speech.

In 2014, outside groups spent $486 million on the Senate races alone, up from $220 million in 2010, the Brennan Center found. Republicans outspent Democrats by about $40 million on the Senate races in 2014, and they reclaimed the Senate majority.

Most of that outside money comes from the super-rich. Super PACs have spent $1 billion on the last three federal election cycles since 2010. Nearly 60 percent of those donations — more than $600 million — were made by just 195 people and their spouses, the Brennan Center found.

The report also examined the 10 richest super PACs from the 2014 Senate election campaign and found that the average contributions were in the five- to six-figure range. All but two of those super PACs received less than one percent of their individual contributions from small donors who gave $200 or less. Seven backed Republican candidates, two Democrats and one, the independent candidate in Kansas, Greg Orman.

Wealthy donors today have “a level of election influence unprecedented in modern American history,” Ian Vandewalker, the report’s author, said in a statement. “Outside money now surpasses even spending by candidates themselves in tight races,” he said.

James Bopp, the intellectual architect behind the Citizens United decision, told FRONTLINE in 2012 that class distinction shouldn’t matter because there’s plenty of money being spent on both sides. “There are rich people on all sides of the political spectrum,” he said.

Campaign spending is only expected to increase in 2016: Last month, Congress passed a measure, buried deep in the government spending bill, that allows individual donors to give 10 times the amount they’re currently allowed to contribute to national parties.

Related Film: Big Sky, Big Money

FRONTLINE’s 2012 film went to the epicenter of the campaign finance debate — rural Montana — to see how the Supreme Court’s Citizens United decision changed campaigns in America.

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