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 January 1 could push the US back into recession.
Earlier, President Barack Obama increased pressure on Republicans to accept a deal, blaming them for the deadlock.
He said their “overriding theme” was protecting tax breaks for the rich.
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.
Sticking points included the fate of expiring Bush-era tax cuts, an estate tax and steep cuts in spending known as the sequester.
If no agreement is reached on Monday, senators are expected to be given the chance to vote on a fallback plan proposed by President Barack Obama.
That would renew tax cuts on earnings under $250,000 and extend unemployment benefits, but does not address the steep spending cuts.
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 December 31.
Analysts say that even if a deal is reached on the fiscal cliff, it will do little to reduce the original problem of the deficit and the government debt limit, raising the prospect of further political in-fighting early in the new year.
Senate Democratic leader Harry Reid and his Republican counterpart Mitch McConnell were locked in negotiations over the weekend.
The two senators appeared to admit not long before the 15:00 deadline that negotiations were at a standstill, with their two parties still divided over core ideological issues about tax and government funding.
Senator Harry 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 recipients, known as “chained CPI”.
Meanwhile Senator McConnell said he had asked Vice-President Joe Biden for help in breaking the deadlock.
“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, Barack 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.
Republicans – some of whom have pledged never to vote for increased taxes – say the deficit is a consequence of excessive government spending.
They want the tax threshold set higher, at around $400,000, and for revenue to be raised by economic growth and cuts in social security and other services states are legally bound to provide.