(Len Andersson/EPA) (Len Andersson/EPA)

George Soros, the billionaire investor, philanthropist and backer of liberal causes, has signed on to be a co-chair of the national finance council for Ready for Hillary, the super PAC mobilizing support for a possible White House bid by Hillary Rodham Clinton.

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

The commitment by Soros – announced by the super PAC Thursday -- represents another heavyweight endorsement for the group, which was started earlier this year by Clinton supporters. Other members of the national finance council, who must contribute or raise $25,000, include Esprit co-founder Susie Tompkins Buell, entrepreneur Jo Ousterhout and attorneys Steve and Amber Mostyn.

Ready for Hillary officials also stress their expanding grassroots support, saying the national network they are developing will prime volunteers and organizers to help Clinton if she decides to run.

“It’s not about pressuring her,” said executive director Adam Parkhomenko. “It’s giving her the time to make the decision, knowing that there’s an operation out there of grassroots supporters who are organizing.”

The super PAC, which says it now has more than one million supporters, gained 30,000 new online sign-ups last weekend alone when Clinton campaigned in Virginia for Democratic gubernatorial candidate Terry McAuliffe, a longtime Clinton friend. Ready for Hillary urged its members to donate to McAuliffe and is organizing supporters to take part in get-out-the-vote efforts for him in northern Virginia. The group plans to similarly back other candidates Clinton may endorse in the coming year.

“Our strategy is to amplify what Hillary is doing and promote the candidates she is out there advocating for,” said spokesman Seth Bringman.

Clinton has not said what she thinks of the efforts of the super PAC, which is prohibited from coordinating with candidates or political parties. But the organization is being backed by longtime Clinton friends and aides, including Craig T. Smith, the former Clinton White House political director, who now serves as the super PAC’s senior adviser.