“George Soros is delighted to join more than one million Americans in supporting Ready for Hillary,” Vachon said. “His support for Ready for Hillary is an extension of his long held belief in the power of grassroots organizing.”

Soros, a longtime Democratic donor to help grass-roots efforts within the party, has been one of the Democrats’ biggest rainmakers over the past decade. His donation also comes at a time when supporters of Clinton, who has yet to say whether she’ll run for president in 2016, are trying to figure out the coexistence of “Ready for Hillary,” which is focusing on grass-roots involvement, and Priorities USA, the former pro-President Obama super PAC that launched devastating attack ads against Mitt Romney last year.