The US government has entered a second shutdown in a matter of weeks after Congress missed the midnight deadline to pass a spending bill.
The White House had already instructed the US federal government to begin preparing for shutdown amid an eleventh hour effort by a Republican deficit hawk to delay a vote on a deal to keep the country running.
Rand Paul took to the Senate floor to argue against proposals that would increase government spending as part of a package of measures to keep the government open beyond midnight.
The senator engaged in an on-again, off-again speech on the Senate floor for more than nine hours, objecting to $300 billion in deficit spending contained in the bill, even as the midnight deadline approached.
His actions meant he single-handedly propelled the government to a shutdown.
"No one will read this bill. Nothing will be reformed. The waste will continue. And government will keep taking your money irresponsibly and adding to a $20 trillion debt," he said.
The day had started with optimism amid expectations that the spending bill would be passed.
It marked a rare bipartisan deal reached between Senate leaders a day earlier allocating an extra $300 billion in spending over the next two years.
It would allow for $165 billion in extra defence spending and $131 billion more for non-military programmes, including health, infrastructure, disaster relief and efforts to tackle an opioid crisis in the country.
Such a deal would have kept the government funded for another two years.
However, it faced opposition from some conservatives, who oppose increased domestic spending, and some liberals, who wanted to hold out and use their leverage over Republicans to win more concessions.
“What you're seeing is recklessness, trying to be passed off as bipartisanship," Mr Paul said.
As the evening wore on the White House's office of management and budget (OMB) began its emergency preparation.
"The office of management and budget is currently preparing for a lapse in appropriations. As we stated earlier today, we support the Bipartisan Budget Act and urge Congress to send it to the president's desk without delay," an OMB official said.
A stop-gap, 16-day funding agreement expired at midnight. It was agreed after a brief, three-day shutdown last month.