The US Senate rejected a new budget measure passed just hours earlier by the House that would also unravel President Barack Obama's signature healthcare law, pushing the US government dangerously close to a shutdown.
The bill would have continued to fund the government after the current fiscal year's budget expires at midnight on Monday in Washington, DC.
But it would also have delayed by one year the "individual mandate" in the Affordable Care Act, which requires all Americans to purchase health insurance. The mandate is scheduled to take effect this year.
Senate Democrats, and the president, have promised to block any such provisions, and indeed the Senate swiftly voted 54-46 to reject the measure. It was the second such vote of the day.
Speaking at a press conference on Monday, Obama called the looming shutdown "entirely preventable," and warned that it would have a "very real economic impact" on Americans. It would "throw a wrench into the gears" of a recovering economy, he added.
Barring any last-minute action, about 800,000 federal workers would be forced off the job without pay. Some critical services such as patrolling the borders, inspecting meat and controlling air traffic would continue.
They will be deliberately bringing the nation to the brink of a government shutdown.
Social Security, the American pension scheme, will continue to pay out benefits, and the Medicare and Medicaid healthcare programmes will continue to function as well.
The latest fiscal fight underscored the deep divide between the Republicans and the Obama administration and its Democratic allies.
Republicans insisted that the healthcare law was costing jobs and driving up healthcare costs.
Obama has said he will not let the law, his chief domestic achievement, be gutted; Democrats say Republicans are obsessed with attacking the overhaul, which is aimed at providing health coverage for millions of uninsured Americans.
Since the last government shutdown 17 years ago, temporary funding bills known as continuing resolutions have been uncontroversial.
But with exchanges set to open on Tuesday where people could shop for healthcare coverage from private insurers, lawmakers from the Republicans' ultraconservative tea-party wing are willing to take the risk in their drive to kill the healthcare law.
The action in Washington on Sunday was limited mainly to TV talk shows and barrages of press releases, as Democrats and Republicans rehearsed arguments for blaming each other if the government in fact closes its doors at midnight on Monday.
"You're going to shut down the government if you can't prevent millions of Americans from getting affordable care,'' said Democratic Representative Chris Van Hollen of Maryland.
"If the Senate stalls until Monday afternoon instead of working today, it would be an act of breathtaking arrogance by the Senate Democratic leadership," said House Speaker John Boehner. "They will be deliberately bringing the nation to the brink of a government shutdown."