US Congressional leaders have one more day to stop the threat of steep tax rises and spending cuts, known as the "fiscal cliff", after talks ended with no deal.
Senators will continue to seek a compromise deal on Monday to send to the House of Representatives.
Failure to reach agreement by 1 January could push the US back into recession.
Earlier President Barack Obama increased pressure on Republicans, blaming them for the deadlock.
He said their "overriding theme" was protecting tax breaks for the rich.
Fallback plan
Talks ended on Sunday with no deal after a day that saw Republican and Democratic senators wrestle over detail and seek to shape a final bill.
The main points of contention remain the fate of expiring Bush-era tax cuts, an estate tax and steep, automatic cuts in government spending.
If no agreement is reached on Monday, senators are expected to be given the chance to vote on a fallback plan proposed by President Obama.
That would renew tax cuts on earnings under $250,000 (£154,000) and extend unemployment benefits, but does not address the spending cuts.
Both the House and Senate are due to convene on Monday in a last minute attempt to bridge the gap between the two sides. The Republican Speaker of the House, John Boehner, has insisted that the Senate act first.
The current stand-off has its roots in a failed 2011 attempt to tackle the government debt limit and budget deficit. Republicans and Democrats agreed then to postpone difficult decisions on spending until the end of 2012, and imposed a threat of compulsory cuts if no deal was reached by 31 December.
Many analysts say the combination of higher taxes and government cut would significantly reduce consumer spending, leading the US economy to fall off the "cliff".
They also say that even if a deal is reached, it will do little to reduce the original problem of the deficit and the government debt limit, raising the prospect of further political infighting early in the new year.
Parties divided
Senate Democratic leader Harry Reid and his Republican counterpart Mitch McConnell were locked in negotiations over the weekend.
The two senators appeared to admit before the 15:00 deadline (20:00 GMT) that negotiations were at a standstill, with their two parties still divided over core issues.
Senator Reid said the Democrats were as yet unable to make a counter-offer to an apparent Republican proposal to slow cost-of-living increases for social security, the state pension scheme.
Meanwhile Senator McConnell said he had asked Vice-President Joe Biden for help in breaking the deadlock late on Sunday.
"I'm concerned with the lack of urgency here. There's far too much at stake," he said. "There is no single issue that remains an impossible sticking point - the sticking point appears to be a willingness, an interest or courage to close the deal."
In his interview with NBC's Meet the Press, broadcast on Sunday, Mr Obama said the priority was to ensure taxes do not rise for middle-class families, saying that would "hurt our economy badly".
"That's something we all agree on. If we can get that done, that takes a big bite out of the 'fiscal cliff'," he said.
There is also debate over where to set the threshold for tax rises. Democrats say the Bush-era tax cuts should be extended for all Americans except the richest - those with annual earnings of more than $250,000 (£155,000).
Republicans - some of whom have pledged never to vote for increased taxes - say the deficit is a consequence of excessive government spending.