Nonprofits Groups and Dark Money in the Presidential Campaign

In the case called Citizens United, the US Supreme Court took the limits off campaign contributions as long as the sources of money were made public. Since then, so-called super PAC's have received a lot of attention and so have their donors. But now it turns out that the super PAC's are being outspent by "social welfare" groups which don't have to reveal where their money is coming from. Kim Barker reports on campaign finance for ProPublica.