US President Donald Trump has threatened a "very long" government shutdown if Democrats do not fund his long-promised border wall.
In a series of tweets, Mr Trump demanded $5.7bn (£4.5bn), which was passed by the House of Representatives, but is expected to fail in the Senate.
"Shutdown today if Democrats do not vote for Border Security!" he tweeted.
If no deal is reached, parts of the US government will begin to close at midnight on Friday.
The Senate is expected to vote at around midday on Friday on a measure that the House approved a day earlier by 217-185.
The Republican president was meeting senators from his own party beforehand, according to the White House, to discuss "the funding bill and the importance of border security".
Any partial shutdown would be the third such closure of federal agencies in 2018.
And if it occurs, it may not be settled until after the New Year, when Democrats take control of the House.
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What did Mr Trump say?
In early morning tweets on Friday, Mr Trump accused Democrats of "trying to belittle the concept of a Wall, calling it old fashioned".
"The fact is there is nothing else's that will work, and that has been true for thousands of years. It's like the wheel, there is nothing better," Mr Trump wrote.
"In Israel the Wall is 99.9% successful," he added. "Will not be any different on our Southern Border!"
"If the Dems vote no, there will be a shutdown that will last for a very long time. People don't want Open Borders and Crime!"
Mr Trump – who will postpone his holiday in Florida in the event of a shutdown, aides say – urged Senate Republican leader Mitch McConnell to invoke the so-called "nuclear option".
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What is the 'nuclear option'?
It would allow the budget to pass by a simple majority of 51 votes, rather than the 60 currently required under Senate rules.
The president's fellow Republicans currently have 51 seats in the 100-seat Senate.
But Mr McConnell has repeatedly refused in the past to invoke such an extreme legislative manoeuvre.
Many Republican senators on Friday made clear their staunch opposition to the "nuclear option".
They warned it would be politically explosive in an upper chamber that prides itself on cross-party comity.
Retiring Republican Senator Jeff Flake of Arizona tweeted: "Deploying the nuclear option would blow that [bipartisanship] up. I will not vote to do it."
Democrat Senator Carl Levin of Michigan said it would be "like allowing the home team to change the rules in the middle of the game".
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What happens if the government closes?
Roughly a quarter of the federal government – including the departments of Homeland Security, Transportation, Agriculture, State, and Justice – will shut down at midnight on Friday if no deal is reached.
National parks and forests will be closed.
But federal programmes on pensions and healthcare will continue to function, as will the military, border patrol, coast guard, federal judiciary, air traffic control and airport security.
The US Postal Service, which is delivering millions of packages before Christmas, will also be unaffected as it is an independent agency.
Federal workers who are deemed "essential" will also remain on the job, but will not receive a pay cheque directly before the Christmas holiday.
What's the other reaction?
"Ugh, are you ruing my life?" said Republican Senator Susan Collins when she was informed by reporters that Mr Trump had promised to veto a budget deal that did not include funding for a wall on US-Mexico border.
Trump supporters angry at wall 'retreat'
Democratic Senator Kirsten Gillibrand called the shutdown "100% avoidable", accusing Mr Trump of "choosing funding for his ineffective and wasteful border wall over what's best for our country".
President Trump said previously that American taxpayers would not have to pay for his wall. Now, he is focused on shutting down the government to force taxpayers to pay $5 billion for an ineffective border wall.
— Senator Cortez Masto (@SenCortezMasto) December 21, 2018
Meanwhile, a US military veteran's grassroots $1bn fundraiser to erect a border wall, and "help President Trump make America safe again", has raised over $11m.
A rival online campaign, called "Ladders to Get Over Trump's Wall", has raised nearly $75,000 in its first day.