WASHINGTON — The Senate on Saturday rejected two Democratic proposals to let tax cuts for the wealthiest Americans expire, forcing the outgoing Democratic majority to compromise with Republicans or risk allowing tax breaks to lapse for virtually everyone at the end of the year.

Efforts quickly shifted to negotiations that would extend the Bush-era tax cuts for all Americans, an outcome that seemed increasingly certain.

The pair of nearly party-line votes – one to preserve the tax cuts for only the first $250,000 of family income, and the other for the first $1 million of income – also represented a final stand for Democrats as the legislative year winds down.

Congress has much to do before its self-imposed deadline of Dec. 17, including ending the standoff on taxes, passing a funding resolution to keep the federal government operating into next year, renewing jobless benefits for millions of unemployed Americans and ratifying an arms treaty with Russia, a top priority for President Obama.

As Senate Democrats staged their tax votes Saturday, Republicans engaged in behind-the-scenes talks with the Obama administration on a compromise plan to extend all the breaks, enacted under President George W. Bush, for two to three years.

Obama had favored the Democratic approach of preserving only the middle-class breaks, but pledged after the votes that he and other negotiators would “roll up our sleeves” starting this weekend to cut a deal.

“With so much at stake, today’s votes cannot be the end of the discussion,” Obama said. “We need to redouble our efforts to resolve this impasse.”

According to the White House, the president told Democratic congressional leaders that he was open to compromise but would oppose even a temporary extension of the Bush-era tax cuts if it did not include an extension of benefits for the unemployed and extensions of the other tax cuts that benefit middle-class families.

A deal on the Bush tax cuts, combined with an extension of unemployment benefits and the Senate ratification of the New START treaty, would represent one of the most significant bipartisan agreements of Obama’s presidency.

The House voted Thursday to end the cuts on income above $250,00 for families and $200,000 for individuals, the plan that Obama has backed.

GOP senators voted unanimously against that proposal Saturday, along with four Democrats – James Webb of Virginia, Joe Manchin of West Virginia, Ben Nelson of Nebraska and Russell Feingold of Wisconsin, as well as Joseph Lieberman, I-Conn. The vote was 53-36, short of the 60 needed to advance.

The proposal by Sen. Charles Schumer, N.Y., to let the cuts lapse for income over $1 million was defeated 53-37. Republicans were united in their opposition, and they were joined by Lieberman, Feingold and Democratic Sens. Jay Rockefeller of West Virginia, Richard Durbin of Illinois and Tom Harkin of Iowa.

Schumer refused to concede defeat. “We’re not giving up,” he said after the votes.

Schumer said some Senate Democrats favor allowing the entire Bush tax package to expire on schedule Dec. 31, forcing the next Congress to resolve the issue in January, once Republicans take control of the House and become a more powerful Senate minority.

Democrats say they are convinced that popular opinion is on their side, and some think voters are so concerned about income disparity and government debt that they will blame Republicans if federal withholding on their paychecks suddenly spikes.

The Saturday showdown revealed a potentially significant new fault line that appears to be developing on Capitol Hill, as lawmakers and the White House adjust to a political landscape radically altered by the midterm elections.

Democrats are using the lame-duck session to push through as many liberal policy priorities as time permits.

Senate Majority Leader Harry Reid, D-Nev., announced Saturday that he would attempt to advance new collective-bargaining rules for firefighters, an overhaul of immigration laws for people who were brought to the United States illegally as children and a repeal of the “don’t ask, don’t tell” restrictions on gays serving openly in the military.

None of those initiatives is expected to advance, but Democratic lawmakers have promised their supporters one final effort before their Senate majority is reduced to 53 from 58.

GOP leaders, meanwhile, appear to be testing out new tactics as they pivot from the opposition role that defined the party’s approach during Obama’s first two years in office.

“I think it’s a healthy sign now that there’s probably been more conversations between the White House and Senate and House Republicans in the last two weeks than in the last two years,” said Senate Minority Leader Mitch McConnell, R-Ky.