Senate campaign spending by groups that do not fully disclose their donors-- known as "dark money"-- has increased over 200% since 2010, when the Supreme Court's decision in Citizens United gutted corporate contribution limits. In the three election cycles since, outside spenders have put $1 billion into Senate elections-- almost half of which, $485 million, was untraceable dark money. In 2014, an estimated 71% of non-party spending in support of the winning candidates was dark money. Many, such as Justice Kennedy, believe disclosure of a politician's backers to be vital for informed voting.