Eighteen incoming members of Congress each got more than $1 million in dark money donations during their recent campaigns, but many more have reason to resent the stealthiest of campaign contributions, a Sunlight Foundation analysis has found.

Dark money represents campaign contributions whose sources never have to be publicly reported. That's because the money is funneled through non-profit entities organized under a section of the tax code that protects them from having to name their donors. These kind of groups -- such as the pro-GOP Crossroads GPS and the pro-Democrat League of Conservation Voters -- have increased their electoral role in the wake of a series of court rulings that opened the door for unlimited corporate and union spending on campaigns. Nonprofit groups made more than $300 million of such donations during the course of the 2012 election cycle, the vast majority to influence the fall races.