The US government has shut down after Congress failed to pass a key budgetary measure on time.
Lawmakers had hoped to approve a new spending bill before federal funding expired at midnight (05:00 GMT).
But Republican Senator Rand Paul ended hopes for a quick vote when he demanded a debate in the chamber on his amendment to maintain spending caps.
In January, a similar failure to pass a bill led to a three-day government shutdown.
Late on Thursday, as Senator Paul continued to delay the vote, the White House said it was preparing for a possible shutdown.
Both the Senate and the House of Representatives need to approve the new two-year spending deal.
The shutdown was essentially guaranteed an hour before the deadline, when the Senate voted for a recess until 00:01.
Despite the delays, the Senate is due to vote on the budgetary measure after 01:00. The House will not vote on the deal until the Senate approves it.
It is not yet clear how Congress will proceed and how public services may be affected on Friday.
The latest deal would raise spending caps by about $300bn (£215bn) – something which Senator Paul insisted he could not support.
Why are budget hawks 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.
In a doom-laden speech, Senator Paul 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.
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 support 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.