One of those swing voters, Sen. Mary Landrieu (D-La.) indicated Sunday on CNN's State of the Union that she may not support a public option.
"I would tend not to," she said. "But, we've got to keep working to find solution."
Meanwhile, moderate Sen. Ben Nelson (D-Neb.) also indicated he may not be willing to support proposals supported by the party's liberal wing.
Even as liberal groups hammer him for not taking a stance on the public option, the senator has apparently received a great deal of feedback from constituents against health care reform, the Wall Street Journal reports.
"I was going to be cautious before," Nelson reportedly said about his vote on health legislation, after a town-hall meeting. "Now I guess I would say I'll be super cautious."
For most liberals, a public option is considered a necessary part of health care reform because, they argue, it would keep down costs by acting as a competitor to the private industry. For that reason, they are closely watching moderate senators like Nelson and Landrieu, who could determine whether Democrats can push through a health care bill without Republican support.
Senate leadership is holding out until Sept. 15 to see if bipartisan negotiators from the Senate Finance Committee can come up with a compromise on health care that would win some Republican votes. Failing that, Democrats on Oct. 15 will be able to use a simple majority to pass some components of health care reform using a procedure called reconciliation.
It is unclear, however, whether some elements of reform -- like the public option -- could pass via reconciliation, since Senate rules dictate that only budgetary items may be considered through the process. Moreover, it is still unclear whether even a simple majority in the Senate supports a public option.
Given the uncertainties at hand, liberals are still pressing some Republicans to work harder for a compromise. The Progressive Change Campaign Committee and Howard Dean's Democracy for America are running a television ad in Iowa and Washington pressuring Sen. Chuck Grassley (R-Iowa) to support a public option. Grassley is one of the three Republicans working on bipartisan negotiations in the Senate.
The ad, shown at left, features Iowa Republican Kevin Shilling, who says there is bipartisan support in their state for a public option.
"When Grassley takes over $2 million from the big health and insurance industries that oppose reform and then says he won't give Iowans the choice of a public option," Shilling says in the ad. "I have to ask, Senator, whose side are you on?"
Sen. Chris Dodd (D-Conn.), who had led health care discussions this summer in the Senate Health, Education, Labor and Pensions Committee, said Sunday that President Obama needs to "step up" and give Democrats more direction on what to support in health care reform.
"I think the president's got to decide to step up and really frame this again for us," Dodd said Sunday on NBC's "Meet the Press."
CBSNews.com Special Report: Health Care