Washington DC - Four months after barely avoiding a catastrophic default, the world's largest economy is once again on the verge of a major fiscal crisis as a government shutdown looms in the US.

Democrats are engaged in a bitter feud with Republican rivals in Congress over spending bills, which, if not passed into law soon, may trigger a government shutdown.

Lawmakers have until midnight on September 30 to reach an agreement before funding for government services is due to dry up.

A shutdown would put at risk the finances of hundreds of thousands of workers at national parks, museums, and other sites operating on federal funding, but it could also carry significant political risk for President Biden as he runs for re-election in 2024.

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House Republicans failed to support the government spending levels agreed to between Biden and Speaker Kevin McCarthy, the top Republican in Congress, that would keep government gears turning, the White House has said.

"A small group of extreme Republicans don't want to live up to the deal, so now everyone in America could be forced to pay the price," Biden said Saturday. "It's time for Republicans to start doing the job America elected them to do."

Tensions are mounting around additional aid for Kyiv after Ukrainian President Volodymyr Zelensky visited Congress Thursday pleading for more weapons to battle Russian forces 18 months into the war.

Both parties in the Senate support the $24 billion aid bill, but a handful of hardline Republicans in the House of Representatives are threatening to block it.

"I am not voting for one single penny to go to a war in Ukraine," congresswoman Marjorie Taylor Greene, a close ally of former president Donald Trump, said in a video posted on X, the former Twitter.

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"I am America First, I work for the United States of America. I work for the American people."

Fellow House Republican Eli Crane echoed that view, writing, "People all over the country are so tired of funding others... We continue to spend and spend and spend, money that we don't have," he said in a video on social media.

Such bluster is putting McCarthy in a bind.

"He's in a very difficult position because the holdouts keep saying to Kevin McCarthy: 'Don't bring bipartisan bills to the floor. We don't want you to use Democrat votes to try to avert a shutdown,'" House Republican Mike Turner told ABC News Sunday talk show This Week.

The budget vote in Congress regularly turns into a standoff, with each party using the prospect of a shutdown to obtain concessions from the other – until a solution is found at the last minute.

But this year, the showdown is exacerbated by new levels of polarization on Capitol Hill.

"Leader McConnell and I are both strongly for aid for Ukraine," Democratic majority leader Chuck Schumer told CNN Friday. "And I believe the majority of the members of both parties in the Senate agree with that."

If no firm agreement is reached, lawmakers could turn to a short-term funding measure, called a continuing resolution, which would offer temporary respite to lawmakers to find common ground.

The shutdown prospect comes just four months after the debt ceiling crisis, during which the US came dangerously close to defaulting on its debt, which could have had disastrous consequences for the American economy and beyond.

As part of the deal averting default, Democrats agreed to limit certain spending in hopes the budget would be approved smoothly. "A deal is a deal," White House Press Secretary Karine Jean-Pierre said, blaming Republicans for the risk of a "needless shutdown."

If the government were to halt operations, low-income families may not receive their food aid checks, air traffic could be disrupted, and national parks could close. Civil servants deemed "non-essential" will be asked to stay home, receiving paychecks only when the problem is resolved.

"I don't want to see a shutdown," House Republican Tony Gonzales told CBS News on Sunday. "But there is no doubt in my mind that the country is headed for a shutdown, and everyone should prepare as such."