(CNN) - MoveOn – which has spent millions this fall on campaigns pressing conservative Democrats to support health care reform – is now calling on members to fight the bill currently being considered by the Senate.
The group has launched a petition drive to push liberal lawmakers like Vermont Sen. Bernie Sanders, an independent who caucuses with Democrats, and Wisconsin Sen. Russ Feingold to block the bill in its current form unless significant changes are made.
Earlier this week, Sanders indicated he was not sure he could back the bill as it now stands.
"(T)here's enormous pressure from all sides to pass a bill quickly, no matter how weak it is," MoveOn organizers said in a message sent to members Friday. "Let's show Bernie and other progressives that we're counting on them to block this version of the bill–and we'll get their backs if they do."
The group wants the final legislation to look more like the House bill, which included a "real public option."
That measure was removed from the bill to win the support of senators like Joe Lieberman of Connecticut, the other independent who caucuses with the Democrats.
"…America needs real health care reform–not a massive giveaway to the insurance companies. Senator Bernie Sanders and other progressives should block this bill until it's fixed." Despite the plan's removal from the Senate bill, "we can still win the health care fight," they write.
On Thursday, two major unions - the AFL-CIO and the Service Employees International Union - issued statements criticizing the revised bill. But neither indicated an intention to oppose the measure.
–CNN Political Producer Rebecca Sinderbrand contributed to this report