The US Senate convened a special session Saturday amid deep divisions over a $1.1 trillion spending bill that lawmakers must pass to avert a government shutdown.
The massive legislation has already been approved by the House of Representatives, and while it is expected to pass the Senate, feuding senators on Friday were unable to get to final passage.
Senate Majority Leader Harry Reid and Minority Leader Mitch McConnell had expressed hope that a deal would be reached for a final vote Friday on the bill funding nearly all federal operations through next September, the end of fiscal 2015.
But conservatives, furious that the legislation failed to roll back President Barack Obama's recent unilateral immigration action to shield millions from deportation, demanded an opportunity to amend the measure.
When Reid objected, Republicans signaled they would push a final vote to Monday.
"Regrettably, a small group of Senate Republicans has determined that it is in their political interests to hold this legislation hostage," Reid told the chamber Saturday, specifically singling out and blaming Senator Ted Cruz of Texas for the delay.
- Obama's immigration order -
The conservative Cruz, widely accused by Democrats of causing last year's 16-day government shutdown, appeared in no hurry to ease passage of the spending bill Friday, giving a fiery floor speech attacking congressional leaders for their failure to stand up to Obama's immigration order.
"Before the United States Senate is a bill that does nothing, absolutely nothing to stop President Obama's illegal and unconstitutional amnesty," Cruz said.
The spending bill funds all agencies of government through September except the Department of Homeland Security, which is tasked with carrying out Obama's immigration order.
DHS funding would last until February, allowing lawmakers in next year's Republican-controlled Congress to take another crack at limiting the immigration plan.
Otherwise the bill leaves Obama's executive order intact.
The House on Friday narrowly passed an extension until Wednesday, but amid the acrimony in the Senate no extension deal was finalized.
- 'Grinding wheels to a stop' -
If there is no agreement by 11:59 pm Saturday to extend funding, the government plunges into shutdown Sunday.
"We are at the mercy of Republican objections," a Democratic leadership aide said.
A Republican aide said Reid's decision to hold votes on several controversial nominees, including Obama's pick for US surgeon general, is also causing Republicans to stall.
But Reid complained that "this isn't about nominations. This is about grinding the wheels of the Senate to a stop."
A bitter budget clash between Republicans and Democrats triggered the October 2013 government shutdown.
Fueled by their anger over the president's health care law, many Republicans refused to accept a budget agreement unless it placed limitations on so-called Obamacare.
The shutdown cost $6.6 million in lost workdays for furloughed federal workers, amounting to $2.0 billion in lost productivity, according to a White House report.
And US national parks ordered closed during the shutdown lost $500 million in visitor spending.