Political spending on congressional elections by 'dark money' groups—organizations that spend money on elections but do not have to disclose the sources of their money — increased dramatically over the past several elections. The vast majority of that increase can be attributed to so-called "social welfare" organizations.
A MapLight analysis of OpenSecrets.org data shows that these groups, considered 501(c)(4) nonprofits, increased spending on congressional elections from $84 million in 2010 to $133 million in 2012. According to U.S. Code they are supposed to be civic leagues "operated exclusively for the promotion of social welfare."
As we move into the 2014 election cycle, the growth in spending by social welfare groups is showing no sign of slowing down. According to Opensecrets.org, social welfare groups have already spent $3.7 million.