Close to half of the money — 41 percent — raised by the groups by the end of February came from just 50 mega-donors and their relatives, according to a Washington Post analysis of federal campaign finance reports.

In all, donors this cycle have given more than $607 million to 2,300 super PACs, which can accept unlimited contributions from individuals and corporations. That means super PAC money is on track to surpass the $828 million that the Center for Responsive Politics found was raised by such groups for the 2012 elections.

The staggering amounts reflect how super PACs are fundraising powerhouses just six years after they came on the scene. The concentration of fundraising power carries echoes of the end of the 19th century, when wealthy interests spent millions helping put former Ohio governor William McKinley in the White House.