The US government shut down at midnight on Friday, meaning thousands of “non-essential” federal workers will be put on leave and not paid until a funding deal is reached.
The shutdown came on the one-year anniversary of Donald Trump’s inauguration as President, and as Republicans enjoy majorities in both the House and Senate.
Scores of federal agencies across the country will be unable to operate until a deal can be agreed.
However, essential employees who deal with public safety and national security will keep working.
Congress scheduled an unusual Saturday session to begin considering a three-week version of the short-term spending measure.
How did it happen?
Republicans in Congress passed stop-gap funding legislation on Thursday but their colleagues in the Senate needed support from 10 Democrats to pass it there. Five voted in favour, but five Republicans opposed the measure and it failed to pass.
The central dispute came over protections for undocumented migrants who were brought to the US as children. Democrats wanted to force through protections from deportation for 700,000 “dreamers” who had previously been covered by the Deferred Action for Childhood Arrivals (Daca) programme.
Mr Trump, who had made strict measures on immigration a cornerstone of his presidential campaign, rejected a bipartisan proposal last week during the now infamous “s***hole countries” meeting.
He said he wanted to include any deal for dreamers in a bigger legislative package that also boosted funding for a wall and tighter security measures along the US border with Mexico.
Mr Schumer met the President on Friday afternoon. One person familiar with the events said the two men agreed to seek a grand deal encapsulating both positions – but that by early evening the agreement was dead, after Mr Trump had spoken with conservative Republicans and heard their objections.
“He did not press his party to accept it,” Mr Schumer claimed later.
What does it mean?
Many thousands of federal workers will not report for work on Monday morning if the impasse is not solved.
That will not include those in so-called essential services like law enforcement, the military, air traffic control and social security – ones that protect “life or human property”.
Even then, not all staff will turn up. Thirteen per cent of workers at the Department of Homeland Security will be furloughed, as will half of those at Health and Human Services, according to contingency plans cited by The New York Times.
Some of those who do work during the shutdown may not be paid until after funding is restored.
CNN warned that some zoos and museums may close while funding is withheld. Special counsel Robert Mueller’s Russia investigation, however, will continue, according to the broadcaster.
Mr Trump’s administration said it planned to keep national parks open with rangers and security guards on duty. The parks were closed during the last shutdown in 2013, upsetting many tourists and resulting in the loss of $500m (£361m) in visitor spending in areas around the parks and at the Smithsonian museums.
How long will it last?
Both parties said they would restart negotiations and the Senate is set to come back into session at noon on Saturday.
Mr McConnell said he would seek a new funding bill to cover the federal government until 8 February, a week less than the failed bill would have done.
Democrats and Republicans both said they wanted a quick agreement.
But both sides may now be even less willing to make concessions, fearing a highly visible political defeat ahead of mid-term congressional elections later this year.
The last shutdown, which took place in 2013 under President Barack Obama, lasted for 16 days. It was overwhelmingly unpopular among voters with 81 per cent disapproving of legislators allowing federal funding to lapse, according to a Washington Post/ABC News poll.
Additional reporting by agencies