The liberal group MoveOn.org is opposing the Senate health care bill and urging Democrats to block it.
In a letter to members Friday, MoveOn.org complained that the legislation has no government-run insurance option and no expansion of Medicare.
It urged its members to sign a petition saying, "American needs real health care reform _ not a massive giveaway to the insurance companies."
The letter also called on members to send a message to Senate progressives such as Bernie Sanders of Vermont, Roland Burris of Illinois and Russ Feingold of Wisconsin to "block this bill until it's fixed."
MoveOn.org says it has 5 million members. Liberals such as Howard Dean have widely criticized the Senate bill.