More than the Kochs: Big donors outside the campaign finance spotlight

Tom Steyer sits in the top spot. In the 2014 election, he gave $73 million to democratic candidates and causes. Steyer is a hedge fund manager and environmentalist behind the NextGen Climate Action super PAC.

The second largest donor in 2014 was Michael Bloomberg, who gave most of his $28 million in contributions to Democratic candidates/causes, but also spent about half a million on Republican candidates.
http://sunlightfoundation.com/blog/2015/10/09/more-than-the-kochs-big-donors-outside-the-campaign-finance-spotlight/