Washington ? The fate of the Medicare prescription drug bill rests with 100 senators after the measure narrowly passed the House, which endured a dusk-to-dawn debate capped by the longest roll call vote in the chamber’s history.

Emboldened Senate opponents of the legislation pledged Saturday to try to stall the bill, citing what they called unfair and partisan tactics employed by Republican leaders in the House to wrest approval of the sweeping Medicare legislation.

President Bush, eager to sign it and promote it on the campaign trail, praised the 220-215 passage in the Republican-controlled House and called on senators to follow suit.

“We’re on the verge of success” of modernizing and strengthening Medicare, Bush said Saturday in a radio address that aired hours after the vote.

Senators began an expected three days of debate soon after Bush spoke, as two Democrats threatened a filibuster to block a vote on the bill.

The legislation’s backers would need 60 votes to close off debate and hold a final vote on the Medicare legislation, which would give 40 million older and disabled Americans a prescription drug benefit and a new option for private health care coverage.

The bill’s backers predicted there would be no repeat of the partisan struggle in the House. “I think you’ll see an entirely different atmosphere in the Senate,” said Sen. John Breaux, D-La., one of the architects of the compromise.

Senate Democratic leader Tom Daschle of South Dakota said he opposed a filibuster to delay a vote on the legislation, even though he plans to vote against the bill.

But after the extraordinary House vote, which GOP leaders held open for nearly three hours while they pursued Republican holdouts, Massachusetts Democratic Sens. Edward M. Kennedy and John Kerry promised a filibuster.

Kennedy compared the House vote to the disputed 2000 presidential election in Florida. “It was a rigged vote,” he told reporters in his Capitol hideaway office. “Give this bill a fair vote in the House of Representatives and I’ll drop my filibuster.”

Senate Majority Leader Bill Frist, R-Tenn., said he would seek a vote Monday to end a filibuster, but he hoped Kennedy would reconsider.

“There is a strong bipartisan majority in this body, in the U.S. Senate, in favor of the Medicare prescription drug legislation,” Frist said.

Republican plans to pass the bill and send it to the president before Thanksgiving were nearly thrown into disarray by the House vote.

As written, the legislation would virtually remake Medicare.

For the first time, the legislation would also require those older Americans with annual incomes over $80,000 to pay higher premiums under Medicare Part B, which covers services outside the hospital.

Additionally, it would establish new tax-preferred health accounts, open to individuals with high-deductible insurance policies.

The tax provision and the requirement for higher premiums were part of an effort to appeal to conservatives who favor transforming Medicare and restraining its cost, yet find creation of the new prescription drug benefit distasteful.

Many Democrats argued that some of the conservative-backed elements of the bill were too dear a price to pay for the drug benefit — particularly a provision creating a limited experiment in direct competition between private plans and traditional Medicare beginning in 2010.

The outcome remained unsettled until just before 6 a.m. Saturday, when Republicans finally overcame a rebellion by conservatives in their own ranks and the overwhelming opposition of Democrats.

Roll call votes in the House customarily last 15 minutes to allow lawmakers ample time to reach the chamber.

“In the end, democracy works,” said Rep. David Hobson, R-Ohio, as weary Republicans marked their overtime victory.

House Democratic leader Nancy Pelosi of California countered, “We won it fair and square and they stole it by hook and crook.”

Dozens of lawmakers, participants and spectators both, waited out the drama of the middle-of-the-night roll call.

Hastert, his lieutenants and Health and Human Services Department Secretary Tommy Thompson shuttled from one GOP holdout to another seeking enough votes to prevail. The president lobbied about a dozen lawmakers by phone from the White House late Friday and early Saturday.

Frist said he awoke twice in the middle of the night to check the progress on C-SPAN. “I got up at 5, saw the vote on the screen and said, ‘Oh my goodness,”‘ he said.

The vote was stuck at 216 to 218 for over an hour, the bill seemingly on its way to defeat before a flurry of last-minute switches.

“I did not want to vote for this bill,” said Rep. Butch Otter, R-Idaho. But he did, as did a handful of late GOP converts.

He said afterward he became convinced that if the measure were defeated, another one would come back to the House floor even less to his liking.

The bill drew the support of 204 Republicans and 16 Democrats, many of whom waited until the bill appeared on the verge of passage in the final moments of the roll call before swinging behind it. Voting no were 189 Democrats, 25 Republicans and 1 independent.