WASHINGTON -- Only a day after Senate Democrats voted to move into a historic debate on overhauling the nation's health care system, key centrists made it clear yesterday that the party is still a long way from delivering on its promise to provide near-universal insurance coverage and contain medical costs.

Faced with the prospect of Republican filibusters at every stage, Democratic leaders can only keep the process moving if they deliver the same kind of total unity they managed to achieve in Saturday's vote to begin debate: Every Democratic senator, plus two independents who caucus with them, supported the key procedural motion that required at least 60 yes votes.

But several of those senators spoke out yesterday to say that they will not prepared to support the health care bill itself unless major changes were made.

Centrist Sen. Ben Nelson, D-Neb., said on ABC's "This Week" that he voted to cut off a Republican filibuster and move into debate after the Thanksgiving recess only because that opened the way to changing the bill put forward by Senate Majority Leader Harry Reid, D-Nev.

If the Reid bill had been before the Senate on Saturday, Mr. Nelson said, he would have voted to block it. "I would have voted 'no' to end debate. ... I would not have let it get off the floor. ... That is the next round," he said.

Meantime, Sen. Joe Lieberman, I- Conn., said on NBC's "Meet the Press" that he voted to end debate only because he wants a chance to amend it. Specifically, he said, he wants to remove the so-called public option provision, under which some consumers would be offered an optional government insurance plan that would compete with private insurers.

"I don't think anyone thinks this bill will pass as it is," Mr. Lieberman said, vowing to filibuster the final version of the bill if the public option remained.

Liberal Democrats, meantime, portrayed Saturday's bill as a major victory.

Sen. Charles Schumer, D-N.Y., said during an appearance on CBS's "Face the Nation" that "the wind is at our back."

On "Meet the Press," Sen. Dick Durbin, D-Ill, said the vote Saturday was a victory for President Barack Obama and for Mr. Reid. "We have a lot of different opinions on our side of the caucus, and we came together last night."

The more liberal senators also defended the current bill. Mr. Schumer said he believed a bill with a centrist public option will pass the Senate in the end with enough support to beat back any filibusters.

Mr. Schumer said that if the bill is not passed, many Americans will find their health care canceled in the coming years because of rising costs to employers. The bill, he said, would preserve private insurance and Medicare while requiring the public option to meet the same requirements as private insurers.

Sen. Dianne Feinstein, D-Calif., said the bill's phased implementation is an asset. If problems develop, she said, Congress can fix the bill -- noting that the "public option" for government-backed health care would not kick in until 2014.

"The bill is incremental. We can watch it. We can change it," she said.

Supporters of the public option would not say whether they were willing to compromise on a government insurance program.

Washington correspondent Daniel Malloy writes the "Pittsburgh On The Potomac" blog exclusively at PG+, a members-only web site of the Pittsburgh Post-Gazette. Our introduction to PG+ gives you all the details.