The US government shut down early Tuesday for the first time since 1996 after lawmakers divided over Obamacare failed to reach an agreement to fund federal agencies through the next fiscal year.
President Barack Obama called it the "height of irresponsibility."
He signed a bill Monday night ensuring military personnel get paid during the shutdown, but an estimated 800,000 federal employees will not have jobs to go to come Tuesday morning.
Government operations ranging from veterans' centers to national parks and most of the nation's space agency are now shuttered.
House Republicans tried three times to tie Obamacare to a government funding bill, but the Democrat-controlled Senate rejected each and every attempt. The first two House bills involved defunding Obamacare, while the third would have delayed the individual mandate of Obamacare for a year and barred the federal government from contributing to the health insurance of the president, lawmakers and staffers.
The final House bill nearly died on a procedural vote after both moderate Republicans and hardliner members of the GOP complained, but Speaker John Boehner was able to keep his caucus in line and pass the measure on a 228-201 vote.
Senators, however, quickly stripped the bill of any mention of Obamacare, just as they had two times before, and sent it back to the House yet again.
“We are not going to mess around with Obamacare," Senate Majority Leader Harry Reid said. "They need to get a life.”
Obama held out hope that Congress would "do the right thing again" in the 11th hour, telling reporters in a surprise news conference Monday afternoon that a government shutdown "doesn't have to happen" and would have a real and lasting impact on the recovering US economy.
"You don't get to extract a ransom for doing your job," he said.
The House and Senate now plan to meet in conference committee in an attempt to hash out their differences over a budget bill.
Reid said the Senate planned to reconvene Tuesday morning.
Meeting with Israeli Prime Minister Benjamin Netanyahu, President Barack Obama told reporters earlier on Monday he was "not at all resigned" to the first government shutdown since the winter of 1995-96.
"The bottom line is that the Senate has passed a bill that keeps the government open, does not have a lot of extraneous issues to it, that allows us to then negotiate a longer-term budget ... but ensures that we're not shutting down the government ... at a time when a lot of families" are just now digging themselves out of hard times, he said.