After an all-day game of legislative pingpong that stretched into the early-morning hours today, Democrats and Republicans failed to reach an agreement on how to keep the government funded, forcing the country into its first government shutdown in nearly two decades.
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"This is a very sad day for our country," Senate Majority Leader Harry Reid said shortly after the government shut down this morning. "This is an unnecessary blow to America."
Congress engaged in a game of political hot potato for the past few days as House Republicans have tried to tie the defunding or delay of Obamacare to the government funding bill, and Senate Democrats have insisted they will only accept a clean continuing resolution.
House Republicans are now pushing for a meeting with Senate negotiators in order to work out a deal that would alter Obamacare and fund the government for six weeks.
"The House has made its position known very clearly," House Speaker John Boehner, R-Ohio, told reporters at a late-night news conference. "We believe that we should fund the government and we think there ought to be basic fairness for all Americans under Obamacare.
"The Senate has continued to reject our offers, but under the Constitution there's a way to resolve this process and that is to go to conference, and talk through your differences."
The House adjourned around 1:30 a.m. and Boehner left the Capitol without an agreement in place. Asked whether he is now prepared to vote on a clean continuing resolution, Boehner maintained that Republicans "are hoping that the Senate will take our offer to go to conference."
"Let us resolve our differences," Boehner said. "The House has voted to keep the government open but we also want basic fairness for all Americans under Obamacare."
The path toward ending the shutdown is uncertain, with neither side signaling it is ready to buckle despite the political weight of a government shutdown. The House is scheduled to reconvene today at 10 a.m. while the Senate is expected to meet at 9:30 a.m.
The Senate Monday rejected the House's attempts to add a one-year delay of the individual mandate two separate times.
"They've lost their minds. They keep trying to do the same thing over and over again," Reid said on the Senate floor Monday night shortly after the House voted on a continuing resolution that would have delayed the implementation of Obamacare.
With less than an hour until a shutdown, House Republican leaders moved to send the budget debate to a conference committee with the Senate to negotiate an agreement on the spending measure.
But Reid rejected the offer, saying the Senate would only negotiate on the budget once a clean continuing resolution was passed.
"We will not go to conference with a gun to our heads," the Nevada Democrat said.
Shortly before midnight, Office of Management and Budget Director Sylvia Mathews Burwell sent a memo to government agencies directing them to "execute plans for an orderly shutdown due to the absence of appropriations."
When the clock struck midnight, Congress had not reached a solution to avert a government shutdown.
Every single government department and agency -- from the United States Postal Service to the Department of Education to the Environmental Protection Agency to NASA -- will feel the immediate effects of the shutdown.
According to government estimates, 800,000 of the more than 2 million federal workers could be furloughed, and the offices that employ them have released contingency plans noting how many employees would be forced to stay home and how many would be "excepted."
The effects will be far-reaching: The Department of Education, for example, would furlough more than 90 percent of its total staff for the first week of a shutdown.
The Centers for Disease Control would not be able to support the seasonal influenza program. Food safety and nutrition activities would also not be supported by the Food and Drug Administration.
The Smithsonian, which employs more than 4,000 people, would have to cut that number down to just 688. The Smithsonian Institute, which includes some of the country's most popular museums, will not be able to keep its doors open to the public.
Workers who will be furloughed will be "hurt greatly," Obama said Monday in a statement at the White House.
"And as a consequence all of us will be hurt greatly, should Congress choose to shut the people's government down," he added.
Obama also warned lawmakers that a shutdown would "throw a wrench into the gears of our economy."
"The idea of putting the American people's hard-earned progress at risk is the height of irresponsibility, and it doesn't have to happen," the president said. "Let me repeat this. It does not have to happen."
U.S. troops were shielded from any damage to their wallets when Obama signed legislation assuring the military would be paid in the in the event of a shutdown.
ABC News' John Parkinson and Mary Bruce contributed to this report.