Sen. Joseph Lieberman (I) of Connecticut has emerged as perhaps the key player in the fate of healthcare reform legislation in the Senate – infuriating many Democrats, who accuse him of acting less out of a sense of principle than a desire to protect insurance companies, a key industry in his state.

But as Senator Lieberman and his fellow moderates push the Senate’s massive health bill toward the right, the hard political fact is that it is liberals, not Lieberman, who may soon have to make a tough decision.

It now appears possible that the legislation might not include anything that resembles a public option – government-run insurance intended to compete with private firms. If that’s the case, will liberals vote for it anyway? After all, many Democrats on the more leftward side of the party have long insisted that the public option is something that a health bill absolutely, positively must have.

“[Senate majority leader] Harry Reid is in just a terrible spot. Do you go you with a bill that in effect offends the base?” says health insurance industry consultant Robert Laszewski.

For Democrats, the problem is that they need to get 60 votes to ensure a filibuster-proof majority for healthcare reform. Their caucus – which includes Lieberman – numbers 60 senators. While they might be able to attract a few Republicans, such as Sen. Olympia Snowe (R) of Maine, their margin for error is essentially nil.

Enter Joe. Lieberman in essence seized control of the healthcare overhaul on Sunday by announcing his opposition to a plan to allow uninsured individuals as young as 55 to buy into Medicare. The Medicare buy-in is a crucial part of a compromise intended to ease the concerns of moderates about the public option. That compromise is now unravelling, as Lieberman made clear that he won't vote for anything that can be construed as government competition for private health firms.

“It will add taxpayer costs. It will add to the deficit. It’s unnecessary,” Lieberman said of the Medicare buy-in option in a broadcast interview on Sunday.

The move has angered liberals, who point out that when Lieberman ran as Al Gore's Democratic vice-presidential candidate in 2000, he supported a buy-in proposal as part of the ticket's health plan.

As recently as three months ago, Lieberman talked favorably of such a proposal in an interview with a Connecticut newspaper, according to a video of the interview circulating on the Web.

"My proposals were to basically expand the existing successful public health insurance programs Medicare and Medicaid," Lieberman told The Connecticut Post.

Those comments made clear the Lieberman's support for buy-in was in the past, said Lieberman spokesman Marshall Wittmann on Monday. Today, the senator believes that the health insurance subsidies contained in the Senate bill render a buy-in program superfluous, said Mr. Wittmann.

"This would be a redundant program that would unnecessarily put the Medicare program in jeopardy," the spokesman said.

The uproar only demonstrates how difficult it will be for Senator Reid to round up 60 votes for a bill that contains anything that smacks of public option competition for private firms.

Many liberals have said they won’t vote for a bill that does not have a public option. In the end, would they really do that?

“I think they are going to do whatever the president asks them to do,” says Mr. Laszewski, author of a popular healthcare blog.

If that’s the case, they will likely vote to approve a bill. President Obama has long said he believes the public option is the best means to keep pressure on insurance firms to control costs, but that it is also not the only means, and not an issue over which the bill should live or die.