The US government is poised for another shutdown after a senator held up a budgetary vote by accusing Congress of "spending us into oblivion".
Hours before federal funding expires, the Senate was hoping to pass the mammoth two-year spending bill.
But Kentucky Senator Rand Paul demanded a debate in the chamber on his amendment to kill the bipartisan deal.
Last month failure to pass a bill led to a three-day government shutdown. The previous one in 2013 lasted 16 days.
This latest budget measure has angered Republican fiscal hawks, while Democrats are upset about the lack of an immigration concession.
What happens next?
Funding to keep the government open runs out at midnight when a one-month spending bill expires.
The White House said it was preparing federal agencies for a shutdown.
The legislation faces an even tougher test in the House of Representatives, though Republican Speaker Paul Ryan said he believes they can muscle it through.
Hopes for a quick vote in the upper chamber on Thursday evening dimmed as Senator Paul stood up.
Why is the senator opposed?
In a doom-laden speech, the Kentucky politician angrily charged his fellow Republicans with fiscal profligacy.
"I ran for office because I was very critical of President Obama's trillion-dollar deficits," he said.
"Now we have Republicans, hand in hand with Democrats, offering us trillion-dollar deficits.
"I can't in all good honesty, in all good faith, just look the other way just because my party is now complicit in the deficits."
This would be "the very definition of hypocrisy", he added.
What's in this bill?
As Senator Paul pointed out, the 650-page spending plan was only unveiled on Wednesday night, so the finer details are unclear.
White House legislative affairs director Marc Short said the package would increase spending by "just shy" of $300bn (£216bn).
The Washington Post puts the figure at half a trillion dollars.
The bill contains $165bn of additional defence spending and $131bn in domestic spending, including funding for healthcare, infrastructure and tackling the US opioid crisis, reports Reuters news agency.
The proposal would raise the US debt ceiling until March 2019.
Why are some Democrats unhappy?
Despite the imprimatur of their Senate leader Chuck Schumer, who says the budget accord will "break the long cycle of spending crises", some Democrats have complained that the bill does not address immigration.
The party's leader in the House of Representatives, Nancy Pelosi, said on Thursday morning she was opposed to the plan, but would not order rank-and-file Democrats to vote against it.
The California congresswoman has called for the bill to include a provision shielding so-called Dreamers, young immigrants who entered the US illegally as children, from deportation.
Her remarks came a day after she told the stories of immigrants for eight hours on the floor of the lower chamber in a record-breaking speech.
Obama-era guarantees for those immigrants were cancelled by US President Donald Trump and are set to become invalid next month.
Illinois representative Luis Gutierrez, one of the leading congressional advocates for immigrants, is urging colleagues to vote against the plan.
"Don't collude with this administration," he said.
Why are some Republicans opposed?
While the spending bill's funding for the Pentagon has delighted the national security wing of the party, fiscal conservatives are up in arms about ramifications for the nation's debt.
Ohio congressman Warren Davidson told National Public Radio: "It's not compassionate to bankrupt America."
Does the US debt of $20tn matter?
Mark Meadows, chairman of the conservative House Freedom Caucus, a group of budget hawks, has called the plan "eye-popping and eyebrow-raising".
Senator Bob Corker of Tennessee said the measure amounts to "doubling down on the irresponsible mentality in Congress of spend-now-pay-later".
What's the White House saying?
Kellyanne Conway, an adviser to the president, told Fox News the deal offers long-term budget certainty and funding for his priorities including infrastructure and military funding.
Earlier this week, Mr Trump said he would "love to see" the US government shutdown if there was no deal on funding for his proposed US-Mexico border wall.
Deficits are already projected to climb because of the Trump administration's $1.5tn tax cuts, which were approved by Congress in December, reports BBC.