Founded by former Vermont Gov. Howard Dean, Democracy for America argues that inclusion of a Medicare-style public option in health-care legislation is "non-negotiable."

MoveOn, a Web-based political action committee that works to elect "progressive" leaders, intended to run commercials over the July 4 holiday criticizing Sen. Kay Hagan, D-N.C., for her silence on the public option. But after she endorsed legislation crafted by Democratic colleagues on the Senate Committee on Health, Education, Labor and Pensions that includes that provision, the group dropped its plans.

"This measure is the heart of health-care reform and is supported by MoveOn's 5 million members as well as the majority of the American people," said MoveOn Executive Director Justin Ruben. "With the support of legislators like Sen. Hagan, we can come closer to our goal of making quality health insurance accessible and affordable for everyone."

Health Care for American Now, a labor-based coalition of 1,000 groups, has organized a petition pressuring Feinstein to support legislation that includes a public option.

"We need a senator who is championing, not nay saying, the need for reform," the petition says. "We're hoping Sen. Feinstein becomes a "champion" for the people of California and stands up for President Obama's health reform."

Richard Kirsch, who runs the coalition, said most of the group's ads are "educational" or focused generally on the need for broad-based change.

"We've been promoting reform and yes, asking members of the public to contact their senators," he said Friday. "It's all in support of reform."

Feinstein said in an interview last week that she does support health reform but has concerns about the cost of legislation and the impact on her home state. She discounted the attacks as unhelpful and counterproductive.

Obama was joined on the call with lawmakers by White House health czar Nancy-Ann DeParle, though he led most of the conversation. DeParle and White House deputy chief of staff Jim Messina have been in intense negotiations with hospital representatives in the hopes of extracting guaranteed spending reductions from the industry.