Webb, who won his seat in 2006 in a cliffhanger race against Republican George Allen, has taken a low-key role in the debate so far. It's far from clear whether he'll vote for the bill in the end, although Senate Majority Leader Harry Reid (D-Nev.) is "confident" that Webb's concerns will be addressed and that he will ultimately back the bill, a Reid spokesman said.

"Jim's gone his own way on several things -- he's independent, and I respect it," said Reid's deputy, Majority Whip Dick Durbin (D-Ill.). "But he's been very honest with us throughout, and I count him as an important and valuable member of our caucus."

Still, his early votes have given party leaders pause. He voted for GOP efforts to send the bill back to the Senate Finance Committee as well as to restore proposed cuts to Medicare -- a move some in the GOP said was intended to keep entitlement programs from being "raided" to pay for the new health care initiatives, a label most of Webb's party strongly rejects.

In an interview, Webb said he's "rather skeptical that those cuts [to Medicare] are sustainable," given the increasing number of people from the baby boom generation eligible for the entitlement program. He also said Medicare Advantage programs have improved services in rural parts of Virginia.

"I'm voting my conscience, which Harry Reid wants all of us to do," Webb said. "I have a lot of concerns about Medicare. I think it's important to express those views on these votes."

But the amendments to restore the proposed cuts failed on the floor -- meaning that whatever bill emerges in the end may very well include the Medicare cuts that worry Webb. Asked if that meant he might oppose the underlying bill, Webb said: "I'll see what the bill looks like when we get to the end of it."