One major sticking point is a provision that would allow Americans to buy a federal-run insurance plan if their state allows it. Moderates say they worry the so-called public option will become a huge and costly entitlement program and that other requirements in the bill could cripple businesses.

"I don't want to fix the problems in our health care system in a way that creates more of an economic crisis," said Lieberman.

The sway held by such a small group of senators has annoyed their more liberal colleagues, who could vote against a final bill if it becomes too watered down.

Sen. Sherrod Brown, D-Ohio, said he didn't think rank-and-file Democrats would feel compelled to go that far. At the same time, Brown warned Democratic leaders not to make too many concessions.

"I don't want four Democratic senators dictating to the other 56 of us and to the rest of the country -- when the public option has this much support -- that (a public option is) not going to be in it," said Brown.

The Senate bill would require most Americans to carry insurance and provide subsidies to those who couldn't afford it. Large companies could incur costs if they did not provide coverage to their work force. The insurance industry would come under significant new regulation under the bill, which would first ease and then ban the practice of denying coverage on the basis of pre-existing medical conditions.

Congressional budget analysts put the legislation's cost at $979 billion over a decade and say it would reduce deficits over the same period while extending coverage to 94 percent of the eligible population.

The House approved its version of the bill earlier this month on a near party-line vote of 220-215.

Sen. Dick Durbin of Illinois, the No. 2 Democrat in the Senate, said the health care bill must be passed by the end of the year so that President Barack Obama and Congress can to shift their attention to the economy and improving employment rates.

Such a timeline also would enable Obama to claim victory on a major domestic priority when he delivers his State of the Union speech in January. With one-third of Senate seats up for election in 2010, politics will factor heavily into the outcome of the debate over health care.

Sen. Michael Bennet, a junior Democrat who will be seeking his first full term next year in Colorado, where many districts lean conservative, said he would support the health care overhaul even if doing so means losing his seat.

"The thing that our working families need more than anything else is to end these double-digit cost increases that they're having every single year with health insurance," Bennet said.

Democratic Sen. Chuck Schumer of New York said he believes there are enough votes to include a public insurance option in the bill as long as states were allowed to opt out. To do so, all 58 Democrats and independent Sens. Lieberman and Bernie Sanders of Vermont would have to support it.

Lieberman and Nelson have said they object to the public option. On Sunday, Nelson said he is open to negotiating the provision; he said he would prefer allowing states to opt into the program, instead of having to remove themselves.

Senate GOP leader Mitch McConnell said the lingering reservations by moderate Democrats indicate that the party's leaders have gone too far. On Saturday, no Republican voted to begin debate on the bill, which they said would cripple industry and drive up costs for the average American.

"I believe there are a number of Democratic senators who do care what the American people think and are not interested in this sort of arrogant approach that everybody sort of shut up and sit down, get out of the way, we know what's best for you," said McConnell.

Brown, Bennet and McConnell appeared on CNN's "State of the Union." Lieberman appeared on NBC's "Meet the Press." Nelson appeared on ABC's "This Week."

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Associated Press writers Lolita C. Baldor and Philip Elliott contributed to this report.

The spectator galleries were full for the unusual Saturday night showdown, and applause broke out briefly when the vote was announced. In a measure of the significance of the moment, senators sat quietly in their seats, standing only when they were called upon to vote.

Republican Sen. George Voinovich of Ohio missed the vote.

In the final minutes of a daylong debate, Majority Leader Harry Reid, D-Nev., accused Republicans of trying to stifle a debate the nation needed.

"Imagine if, instead of debating whether to abolish slavery, instead of debating whether giving women and minorities the right to vote, those who disagreed had muted discussion and killed any vote," he said.

The Republican leader, Sen. Mitch McConnell of Kentucky, said the vote was anything but procedural -- casting it as a referendum on the bill itself, which he said would raise taxes, cut Medicare and create a "massive and unsustainable debt."

For all the drama, the result of the Saturday night showdown had been sealed a few hours earlier, when two final Democratic holdouts, Sens. Mary Landrieu of Louisiana and Blanche Lincoln of Arkansas, announced they would join in clearing the way for a full debate.

"It is clear to me that doing nothing is not an option," said Landrieu, who won $100 million in the legislation to help her state pay the costs of health care for the poor.

Lincoln, who faces a tough re-election next year, said the evening vote will "mark the beginning of consideration of this bill by the U.S. Senate, not the end."

Both stressed they were not committing in advance to vote for the bill that ultimately emerges from next month's debate. Even so, their announcements marked a major victory for Reid and the White House in a year-end drive to enact the most sweeping changes to the nation's health care system in a half-century or more.

The legislation would require most Americans to carry insurance and provide subsidies to those who couldn't afford it. Large companies could incur costs if they did not provide coverage to their workforce. The insurance industry would come under significant new regulation under the bill, which would first ease and then ban the practice of denying coverage on the basis of pre-existing medical conditions.

Congressional budget analysts put the legislation's cost at $979 billion over a decade and said it would reduce deficits over the same period while extending coverage to 94 percent of the eligible population.

But this is just one procedural vote. The more daunting hurdle of the cloture vote to break a filibuster and hold an up-or-down vote on the bill itself still lies ahead, and there Reid may have serious trouble, especially if a plan to create a government-run insurance provider -- the public option -- remains in it.

Sen. Joe Lieberman, I-Conn., who is a member of the Democratic caucus, has already said he'll vote to filibuster a bill that contains any form of the public option. (He is voting for cloture tonight, but not, he says, the next time around.) And on Saturday, Lincoln too threatened to support a filibuster of the legislation if it includes the public option.

Both will be tough nuts for Reid to crack. Lieberman's not up for re-election next year, and has already been taunting liberals by saying he's not afraid of possible retribution. Lincoln, on the other hand, is up for re-election -- and that's the problem. She's seriously vulnerable, and is looking at polling numbers that seem to indicate voting with her party to support a public plan would only put her in a more precarious position.