The US Congress faced a deadline on Friday to agree to a fresh round of Covid-19 aid as part of a sweeping government funding bill, pass a third stopgap spending bill so negotiations can continue, or let the government shut down at midnight.
After months of partisan finger-pointing and inaction, Republicans and Democrats have been negotiating intensely this week on what is expected to be a $900 billion piece of legislation to provide relief to a country struggling with a pandemic that has killed nearly 309,000 Americans.
They have reported progress, but enough differences remained by late on Thursday that talks looked likely to stretch into the weekend.
Multiple lawmakers floated the possibility the federal government would run out of money early Saturday morning, if Congress is unable to pass a temporary government funding bill before Friday at midnight.
Republican Senate majority leader Mitch McConnell said the chamber would remain in session through the weekend if necessary, citing the need for urgency.
"The senate is not going anywhere until we have Covid relief out the door," he said, adding that senators would vote throughout the weekend on more of outgoing Republican President Donald Trump's nominations.
The prospect of a government shutdown increases pressure to come up with a relief plan. A shutdown could force thousands of people out of work and disrupt services at a time of high unemployment and uncertainty about distribution of coronavirus vaccines.
A House of Representatives Democratic aide familiar with the negotiations said there was confidence the Democratic-run House could meet the midnight Friday deadline.
House Speaker Nancy Pelosi told reporters negotiators "were doing fine." When asked if a relief plan would pass the House on Friday, she said only, "We'll see, we'll see."
The coronavirus legislation is expected to include stimulus checks of about $600, extended unemployment benefits, help for states distributing the vaccine, and assistance for small businesses struggling through the pandemic.
Members of Congress said they were being spurred to action by an alarming increase in hospitalisations and deaths due to the pandemic. The US coronavirus death toll is by far the world's highest and many Americans - who do not receive government aid that is automatic in many other nations - are struggling.
Republican Senator Rob Portman pointed to growing lines of unemployed Americans at food banks. "Something's going on here folks... people waiting five, six hours for a box of food."
Republicans also have a wary eye on the impact inaction might have on a pair of January 5 runoffs in Georgia, which will decide whether their party maintains control of the Senate for the next two years or hands it over to Democrats.
House and Senate leaders are negotiating a $900bn bill that would be attached to a $1.4 trillion measure to fund federal programmes through September 2021. They hope to pass both in time to avoid a shutdown.
Obstacles to a deal include differences over a Federal Reserve emergency lending programme, how to handle eviction prevention, food aid for the poor, and reimbursements to local governments for expenses like personal protective equipment for schools.