Republicans threatened to delay Senate business with a health care read-a-thon this weekend, as Democrats kept searching for 60 votes to advance President Barack Obama's signature issue and a forecast of heavy snow added to the list of complications.

At a news conference Friday in the Capitol, Republican Leader Mitch McConnell of Kentucky and Sen. John McCain, R-Ariz., accused Democrats of trying to ram the health care bill through with dozens of changes as yet unseen, and promised to do everything in their power to prevent it.

"I think we've made it rather clear we're not going to expedite consideration of the health care bill," McConnell said.

Added McCain: "I don't think it would be outrageous to ask for a bill that we haven't seen to be read."

Democrats, after a week of bitter intra-party disagreements over the compromises Senate leaders made to keep the bill moving, were still trying to come up with the 60 votes needed to overcome unanimous Republican opposition. Leaders kept negotiating with the only known Democratic holdout, Sen. Ben Nelson of Nebraska, on tough abortion restrictions that he would accept.

Meanwhile, as the weather forecast called for up 20 inches of snow in a city easily paralyzed by much less, senators were put on notice they could be needed to vote on procedural questions before 8 a.m. EST Saturday.

Majority Leader Harry Reid, D-Nev., is expected to release a package of changes to the bill that could run to hundreds of pages. Since those changes were negotiated behind closed doors, McCain said it's reasonable to demand that the package be read aloud on the Senate floor. However, such changes in major legislation are commonplace and senators usually waive reading them on the floor.

Democrats were preparing for the read-a-thon. "We have to be in position so we can spend most of (Saturday) reading the bill, because the Republicans insist on it," said Reid spokesman Jim Manley. It would be the third straight weekend the Senate has been in session, mostly on account of the health care bill.

All eyes were on the most conservative Democrat in the Senate, Nebraska's Nelson, whose primary concern is that abortion funding restrictions in the bill are too lax. Nelson indicated Thursday he still was not happy but would keep talking with Reid.

Nelson was vague about his intentions, telling reporters, "I hope we're getting closer" to agreement.

Any hopes the bill's supporters had of a Republican casting a critical 60th vote vanished when Sen. Olympia Snowe of Maine said after a meeting with Obama that the Democrats' timetable for a pre-Christmas vote was "totally unrealistic."

Liberals are furious over the compromises Reid made.

Gone is a government insurance plan modeled on Medicare. So is the fallback, the option of allowing aging baby boomers to buy into Medicare. The major benefits of the bill won't start for three or four years, and then they'll be delivered through private insurance companies. Former Democratic chairman Howard Dean urged its defeat.

Former President Bill Clinton came to the defense of the Senate bill. Clinton, whose ambitions were humbled by the collapse of his own health care remake, reminded Democrats that political pros don't let the perfect become the enemy of the good.

"Take it from someone who knows: These chances don't come around every day," Clinton said in a statement. "Allowing this effort to fall short now would be a colossal blunder _ both politically for our party, and far more important, for the physical, fiscal and economic health of our country."

Overall, the legislation is designed to extend coverage to millions who lack it, ban insurance company practices such as denying coverage because of pre-existing conditions and try to slow rising health care costs.

The bill would require most Americans to purchase insurance, and it includes hundreds of billions of dollars in subsidies to help lower- and middle-class families afford it. An additional 30 million people would be covered.

Politically powerful labor unions have panned the Senate bill but stopped short of calling for its demise, saying they hoped lawmakers ultimately would improve it.

The AFL-CIO, the nation's largest labor federation, and the Service Employees International Union both expressed deep disappointment. The Senate bill includes a tax on high-cost insurance plans that unions fear will hurt their members.

SEIU president Andy Stern scolded Obama, saying the president should remember his campaign promise to bring change to America. AFL-CIO President Richard Trumka said the Senate bill "bends toward the insurance industry."

Both union chiefs said they hoped the bill that finally emerges from Congress would reflect the House-passed measure, which incorporated a government insurance option and omitted the insurance tax.

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Associated Press writers David Espo, Erica Werner and Charles Babington contributed to this report.

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Service Employees International Union: http://www.seiu.org