WASHINGTON (NYTIMES) - US Senators failed on Sunday (Jan 21) to reach an agreement to end the government shutdown, ensuring that hundreds of thousands of federal employees would be furloughed on Monday morning even as the outlines of a potential compromise came into focus.

For much of Sunday, feverish work by a bipartisan group of senators offered a reason for cautious optimism that a deal could be reached soon. By Sunday night, Senator Mitch McConnell, the Senate majority leader, moved to delay until noon Monday (1am Tuesday Singapore time) a procedural vote on a temporary spending Bill - a signal that talks were progressing.

In a gesture to the bipartisan group as it seeks assurances that the Senate will address the fate of hundreds of thousands of young unauthorised immigrants, McConnell said he intended to move ahead with immigration legislation next month if the issue had not been resolved by then.

"It would be my intention to proceed to legislation that would address Daca, border security and related issues," McConnell said, adding: "It is also my intention take up legislation on increased defence spending, disaster relief and other important matters" then. He was referring to the Obama-era Deferred Action on Childhood Arrivals programme, which protects an estimated 700,000 "Dreamers" - undocumented immigrants who arrived as children - from deportation.

Democrats have been insisting that any deal must include legislation protecting "Dreamers".

Senate Democrats gave no immediate sign that they would get on board with the temporary spending Bill, leaving open the possibility of another failed vote on Monday that could further deepen the partisan divide in the Chamber. Any deal would most likely need the support of at least a dozen Senate Democrats, since the Chamber's procedural rules require 60 votes in the 100-seat Chamber.

"We have yet to reach an agreement on a path forward that is acceptable to both sides," the Democratic leader, Senator Chuck Schumer of New York, said after McConnell's remarks.

The best hope for a breakthrough appeared to reside with the group of about 20 senators from both parties who met throughout the weekend to try to hammer out a compromise to present to McConnell and Schumer.

The group was discussing a plan in which the government would stay open through early February, to be coupled with a promise that the Senate would tackle the issue of immigration in the coming weeks. Several members signalled optimism on Sunday afternoon, but it remained to be seen if they could nudge Schumer and McConnell to reach an agreement, particularly over what such a promise might look like.

"There are, I think, people from both parties of good will who want to have a framework for us to move forward to address all of these issues," Democratic Senator Chris Coons said on Sunday evening. "But at this point, it is in the hands of leadership, and I really hope they are going to find a way forward."

A major lingering question was how a compromise might pave the way for passage of legislation to protect Dreamers. Their status is in jeopardy after President Donald Trump moved last year to end Daca, which shields them from deportation. Trump gave Congress until early March to find a resolution to the issue.

For as long as the government is closed, the White House has said it will not entertain demands on immigration. Senators in the bipartisan group proceeded anyway in discussing a compromise in which there would be some kind of promise that the Senate would address immigration in the coming weeks.

"We want to see a commitment to take up immigration, with a belief that we'll get to a good result for Dreamers," said Democratic Senator Tim Kaine.

Republican Senator Jeff Flake said it was best for Trump to let the Senate work out its own solution.

"I just don't think it helps for him to be involved at all right now," he said.

The talk of promised action on immigration raised its own questions, including whether a pledge from McConnell would be a significant enough assurance for Senate Democrats who are worried about the fate of the Dreamers. For instance, a promise of a Senate vote on a stand-alone immigration Bill would still leave the possibility that the measure would die in the House, potentially leaving the Daca issue unresolved.

In a reflection of the frustration on both sides, Trump, who has largely stayed out of negotiations since the government shut down early Saturday, began the day by advising Senate Republicans to use the "nuclear option" to change Senate rules and cut Democrats out of the process if necessary.

"If stalemate continues, Republicans should go to 51% (Nuclear Option) and vote on real, long term budget, no C.R.'s!" he wrote on Twitter, adding an attack on Democrats.

Leaders from both parties quickly dismissed the idea, but Democrats wasted no time in pointing the finger back at Trump.

Schumer's eleventh-hour negotiations with Trump on Friday have proven to be a focal point for both parties as they cast blame. On Sunday, Schumer said that during the White House meeting, Trump had "picked a number" that he wanted in order to fund a border wall and that Schumer had accepted in exchange for protections for Dreamers. Hours later, he said, the White House walked away from a tentative agreement.

"I essentially agreed to give the President something he has said he wants, in exchange for something we both want," Schumer said on the Senate floor. "The President must take 'yes' for an answer. Until he does, it's the Trump shutdown."

McConnell has proposed a temporary spending Bill that would expire on Feb 8, about a week earlier than the date set in a Bill passed last week by the House.

The proposed Feb 8 expiration date was designed to at least partially accommodate a demand by Democrats, who are mindful of preserving what leverage they have, that any temporary funding extension be shorter than the House proposal. But the more significant piece of any possible deal to end the shutdown would be what other strings are attached - particularly when it comes to the politically contentious issue of immigration.

Senators in the bipartisan group made clear they were eager to bring about an end to the crisis, mindful that they were working against the clock on Sunday. "Resolution gets more difficult the longer we wait," said Democratic Senator Heidi Heitkamp.