Economists have warned that without action by Congress, the tax increases and spending cuts that technically took effect with the turn of the new year at midnight could send the economy into recession.
The Senate-passed bill was designed to prevent that while providing for tax increases at upper incomes, as Obama campaigned for in his successful bid for a second term.
The non-partisan Congressional Budget Office said the measure would add nearly $4 trillion over a decade to federal deficits, a calculation that assumed taxes would otherwise have risen on taxpayers at all income levels. There was little or no evident concern among Republicans on that point, presumably because of their belief that tax cuts pay for themselves by expanding economic growth and do not cause deficits to rise.
The relative paucity of spending cuts was a sticking point with many House Republicans. Among other items, the extension of unemployment benefits costs $30 billion, and is not offset by savings elsewhere.
Others said unhappiness over spending outweighed fears that the financial markets will plunge on Wednesday if the fiscal cliff hasn't been averted.
"There's a concern about the markets, but there's a bigger concern, which is getting this right, which is something we haven't been very good at over the past two years," said Rep. Steve LaTourette of Ohio.
House Democrats met privately with Biden for their review of the measure, and the party's leader, Rep. Nancy Pelosi of California, said afterward that Boehner should permit a vote.
"That is what we expect. That is what the American people deserve," she said.
For all the struggle involved in the legislation, even its passage would merely clear the way for another round of controversy almost as soon as the new Congress convenes.
With the Treasury expected to need an expansion in borrowing authority by early spring, and funding authority for most government programs set to expire in late March, Republicans have made it clear they intend to use those events as leverage with the administration to win savings from Medicare and other government benefit programs.
McConnell said as much moments before the 2 a.m. Tuesday vote in the Senate — two hours after the advertised "cliff" deadline.
"We've taken care of the revenue side of this debate. Now it's time to get serious about reducing Washington's out-of-control spending," he said. "That's a debate the American people want. It's the debate we'll have next. And it's a debate Republicans are ready for."
The 89-8 vote in the Senate was unexpectedly lopsided.
Despite grumbling from liberals that Obama had given way too much in the bargaining, only three Democrats opposed the measure.
Among the Republican supporters were Sen. Pat Toomey of Pennsylvania, an ardent opponent of tax increases, as well as Sen. Ron Johnson of Wisconsin, elected to his seat two years ago with tea party support.
It marked the first time in two decades that Republicans willingly supported higher taxes, in this case on incomes over $400,000 for individuals and $450,000 for couples. Taxes also would rise on estates greater than $5 million in size, and on capital gains and dividend income made by the wealthy.
Associated Press writers Andrew Taylor, Larry Margasak and Julie Pace contributed to this story.