The impact of the shutdown that began at midnight Friday has been largely limited so far, closing sites like New York's Statue of Liberty, but the effect will be acute if the stalemate runs into Monday.
Republicans and Democrats have traded bitter recriminations over who is to blame for the failure to pass a stop-gap funding measure by a January 20 deadline, a year to the day since Donald Trump took office as US president.
Highlighting the deep political polarization, crowds estimated to number in the hundreds of thousands took to the streets of major US cities Saturday to march against the president and his policies.
Top Senate Republican Mitch McConnell on Saturday set a key vote for a funding measure for 1:00 am (0600 GMT) Monday, with both houses of Congress set to reconvene Sunday.
"I assure you we will have the vote at 1:00 am on Monday, unless there is a desire to have it sooner," he said in a statement.
At the heart of the dispute is the thorny issue of undocumented immigration.
Democrats have accused Republicans of poisoning chances of a deal and pandering to Trump's populist base by refusing to fund a program that protects 700,000 "Dreamers" -- undocumented immigrants who arrived as children -- from deportation.
Trump, in return, has said Democrats are "far more concerned with Illegal Immigrants than they are with our great Military or Safety at our dangerous Southern Border."
The shutdown's effects meanwhile are set to intensify.
Essential federal services and military activity are continuing, but even active duty troops will not be paid until a deal is reached to reopen the US government.
There have been four government shutdowns since 1990. In the last one in 2013, more than 800,000 government workers were put on temporary leave.
"We're just in a holding pattern. We just have to wait and see. It's scary," Noelle Joll, a 50-year-old furloughed US government employee, told AFP in Washington.
A deal had appeared likely on Friday afternoon, when Trump -- who has touted himself as a master negotiator -- seemed to be close to an agreement with Democratic Senate minority leader Chuck Schumer on protecting Dreamers.
Congress reconvened for a rare Saturday session, where leaders of both sides were meant to hammer out their differences to prevent the shutdown from stretching into Monday. Instead, they traded accusations of responsibility for the shutdown.
Schumer said trying to negotiate with Trump "was like negotiating with Jell-O."
"It's impossible to negotiate with a constantly moving target," he said. "President Trump is so mercurial it's been impossible to get him to agree to anything."
Meanwhile, McConnell said Schumer "took the extraordinary step" of preventing the legislation from passing and thus "plunging the country into this totally avoidable mess."
Republicans have a tenuous one-seat majority in the Senate, and on Friday needed to lure some Democrats to their side to get a 60 vote supermajority to bring the motion forward. They fell ten votes short.
The measure brought to Congress would have extended federal funding until February 16 and reauthorized for six years a health insurance program for poor children -- a long-time Democratic objective.
But it left out any action on the Deferred Action on Childhood Arrivals program, known as DACA, that affects Dreamers.
White House officials insisted there was no urgency to fix DACA, which expires March 5.
As US lawmakers wrangled over government funding, protesters turned out in cities including Los Angeles, New York and Washington to express their opposition to Trump, and their support for women's rights.
Protestors hoisted placards with messages including "Fight like a girl" and "A woman's place is in the White House" and "Elect a clown, expect a circus."
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)