Setting up a high-stakes political showdown, the US House votes Saturday on a Republican plan that keeps government open through mid-December but delays implementation of President Barack Obama's health care law.
But Democrats made it clear that such a move will almost certainly result in a shutdown of federal agencies after the fiscal year ends Monday night.
House Speaker John Boehner gathered his caucus and convened a rare Saturday session as Congress struggled to break a funding impasse that, if unresolved, would require hundreds of thousands of federal workers to stay home.
More than a million US military personnel would remain on duty, but with no pay until a spending deal is reached.
Under pressure from his party's far-right conservative wing, Boehner doubled down on his caucus's bid to stop Obama's signature domestic achievement, vowing to send a bill back to the Senate but with little time for legislative action to avoid a shutdown.
If Republicans vote in unison as expected, the House will approve a measure "that will keep the government open and stop as much of the president's health care law as possible," Boehner said in a statement.
"We will do our job and send this bill over, and then it's up to the Senate to pass it and stop a government shutdown."
The House had earlier passed a temporary budget bill known as a continuing resolution that included a provision defunding so-called "Obamacare" altogether.
The Democratic-led Senate stripped that part out and sent a clean CR back to the House. Instead of passing that measure, the House will amend the bill with a one-year Obamacare delay and repeal of an unpopular medical device tax.
"This is exactly what we wanted," conservative Congressman Raul Labrador told reporters.
But a furious Democratic Senate Majority Leader Harry Reid attacked the move as "pointless" brinkmanship that could end in economic crisis.
"To be absolutely clear, the Senate will reject both the one-year delay of the Affordable Care Act and the repeal of the medical device tax," Reid said in a statement.
"The American people will not be extorted by Tea Party anarchists."
Driving the point home, a Senate Democratic aide said it was highly unlikely the chamber comes into session before Monday.
"We are not playing games," he told AFP. "House Republicans' only way out is to pass the Senate's clean CR or shut down the government."
House Democrats, largely powerless to prevent passage of Republican-backed legislation in the lower chamber, expressed worry that the US government would suffer the same fate as a Washington deadlock in late 1995 that resulted in a 21-day workstop.
"I think we are on our way" to a shutdown, said Congressman John Lewis, a 27-year House veteran. "We're closer now than ever before."
Despite obvious recent divisions among Republicans over the path forward, lawmakers exited Boehner's meeting insisting they were of one mind in pegging an Obamacare delay to the temporary spending bill.
And they charged that Democrats would be on the hook for any government closure, not Republicans.
"Harry Reid will be shutting down the government if he doesn't accept this pretty sensible solution to the debate that we're having right now," Labrador said.
Boehner and fellow House leaders had struggled to stake out a position palatable to their divided members.
A chorus of criticism -- including from Obama as well as some centrist Republicans in the Senate -- accused Tea Party-backed lawmakers of unwisely threatening a shutdown if they did not get their way.
"How dare you presume a failure?" Congressman Darrell Issa boomed to a reporter who asked what would happen when the Senate rejects the House measure.
"We continue to anticipate that there's an opportunity for sensible compromise."
Congress has less than 60 hours to strike a deal that keeps government open, but the ping-ponging of legislation is making that unlikely.
As if anticipating a possible shutdown, Boehner said the House would vote on a separate measure "that ensures our troops get paid, no matter what."