The US government was on the brink of shutting down Sunday after the House of Representatives approved a Republican bill seeking to delay President Barack Obama's health care law.
Congress now has less than 48 hours to strike a deal that keeps the government open, but the ping-ponging of legislation is making that unlikely.
Obama has threatened to veto any bill that undercuts his sweeping health overhaul.
And Democratic Senate Majority Leader Harry Reid says his chamber, which must approve the House measure for it to pass, will reject the bill.
"Tomorrow, the Senate will do exactly what we said we would do and reject these measures," Reid spokesman Adam Jentleson said in rejecting House Republicans' "empty political stunts."
"At that point, Republicans will be faced with the same choice they have always faced: put the Senate's clean funding bill on the floor and let it pass with bipartisan votes, or force a Republican government shutdown."
The House measure funds the government at current levels until mid-December, but also includes a one-year delay of the president's signature health reforms - dubbed Obamacare - and the repeal of a tax on medical devices.
The impasse means that the federal government is now dramatically closer to its first shutdown in 17 years, which would require hundreds of thousands of federal workers to stay home.
After hours of raucous debate, the Republican-controlled House approved its own measure just after midnight Saturday, voting largely along party lines.
Republican leaders set off a political firestorm when they announced that their stopgap federal spending bill sought to delay implementation of the health care law by one year.
The White House sharply rebuked the move, and warned it was a step toward shuttering federal agencies once the fiscal year ends Monday night.
Republican House Speaker John Boehner nevertheless plowed ahead with the strategy, convening a rare Saturday session as Congress struggled to break the funding gridlock.
Under pressure from his Republican Party's most conservative wing, Boehner doubled down on his caucus's bid to stop Obamacare.
"The American people will not be extorted by Tea Party anarchists," said Reid, referring to the ultra-conservative faction of Republicans.
He derided the House measure as "pointless" brinkmanship that could end in economic crisis.
Driving the point home, a Senate Democratic aide told AFP that it was "highly unlikely" the chamber would be in session before Monday.
But Boehner warned that if the Senate waits until hours before the deadline to vote, "it would be an act of breathtaking arrogance by the Senate Democratic leadership."
'Hijacked' by 'extreme folks'
Given the Senate's likely rejection of the House bill in the waning hours of the fiscal year, a Republican aide acknowledged that a temporary shutdown was the likeliest scenario.
House Democrats, a minority force and largely powerless to stop the Republican legislation, worried that this was a repeat of the 1995-1996 crisis, when a budget deadlock resulted in a 21-day federal work stop.
Democratic Representative David Scott said what was occurring was nothing less than "a shutdown being ordered by the Republican Party."
"You have been hijacked by a small group of extreme folks who simply hate this president," Scott said, breaking protocol as he addressed Republican House members directly.
"The American people are never going to forget that it was you who shut down the government."
A chorus of criticism, which included centrist Republican senators, accused Tea Party-backed lawmakers of unwisely threatening a shutdown if they did not get their way.
Former President Bill Clinton decried the Republican strategy and backed Obama's no-compromise stance.
"If I were the president I wouldn't negotiate over these cuts," Clinton told ABC's "This Week" program.
"They're taking food off the table of low-income working people while they leave all the agriculture subsidies for high-income farmers... It's chilling to me."
Many conservatives were unapologetic about seeking to halt a health care law they insist is not ready for prime time.
"I'm a free-market guy, and I truly believe that Obamacare could be the linchpin in shifting America over into an almost irreversible socialist economy," Congressman Trent Franks said.
As if anticipating a possible shutdown, House Republicans introduced a separate measure that would ensure US troops get paid in the event of a work stop.
The measure passed unanimously.