Administration officials insisted that government contractors and state governments will begin receiving word quickly about programs that must be reduced or terminated. The Defense Department is planning to give notice to 800,000 civilian workers on Wednesday that furloughs might be necessary if the automatic cuts go into effect.
Amy Brundage, a White House spokeswoman, said: "Whether these impacts are felt immediately or in the near future, they are already having negative effects on the economy. And there are Americans who are working today who could lose their jobs if these cuts go into effect."
But officials conceded that day care centers are almost certainly not going to be padlocked on March 1. Border patrols will be staffed throughout that day and the days to come. Federal agents will continue to conduct investigations, and criminals will not immediately be "let go," as Mr. Obama suggested.
"The scheduling will depend on what the workload is, what the cases are, what can wait," said Nanda Chitre, a spokeswoman for the Department of Justice. "There's going to be impact all across law enforcement. But we've tried to give as much flexibility as possible."
In recent years a gridlocked Washington has seemed to need the threat of drop-dead deadlines and potentially grave consequences — workers laid off, parks and monuments closing, services disappearing overnight, federal benefit checks delayed — to get anything done.
Think of the last-minute deal to avert the "fiscal cliff" last month, or the one that was struck in 2011 to raise the nation's debt ceiling just as the possibility of default loomed.
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Now, with both Democrats and Republicans hoping to avert the automatic cuts, which are known as the sequester, both sides are emphasizing their immediacy, with Mr. Obama warning on Tuesday that "just 10 days from now, Congress might allow a series of automatic, severe budget cuts to take place."
But even if March 1 arrives without a deal to avert the cuts, it may be some time before many take effect. White House officials have said that the planned cuts would take $85 billion out of the budget this year. But the actual impact of cuts felt this year might be only about half that much, the nonpartisan Congressional Budget Office estimated in a recent blog post.
The office wrote that "discretionary outlays will drop by $35 billion and mandatory spending will be reduced by $9 billion this year as a direct result of those procedures; additional reductions in outlays attributable to the cuts in 2013 funding will occur in later years."
In the short term, officials at a variety of agencies said, the automatic cuts will take some time to put in place as officials readjust their priorities.
Officials at the Department of Homeland Security said that travelers should not notice any significant reductions in Transportation Security Agency staffing at airports in the first few days after the automatic cuts. And the number of air traffic controllers will not immediately be reduced, they said.
"Lines/wait times will increase as reductions to overtime and the inability to backfill positions for attrition occur," a Homeland Security official said.
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Bill Hall, a spokesman for the Department of Health and Human Services, said he did not have details about how child care services would be immediately affected. But he said the longer-term impact might be greater.
"My guess is that this is impacting the spending over a period of time," Mr. Hall said. "I can't say how each one is going to work. I think there's still a lot of work being done."
Ms. Chitre said that the Bureau of Prisons, which houses 218,000 federal prisoners, does not intend to let anyone go on March 1 because of the cuts. She said that bureau personnel might be furloughed and that vocational education programs and others might be curtailed.
As an example, she said the United States Marshals Service would face the likelihood of furloughs, but she added: "They will continue prisoner transfers and protect the courts. They are going to have to do prioritizing."
White House officials said they still believe that the automatic cuts will be severely disruptive right away. And they said decisions to cancel or reduce programs might not be able to be reversed a month or two later.
In his remarks, Mr. Obama said repeatedly that "this is not an abstraction." He added: "There are people whose livelihoods are at stake. There are communities that are going to be impacted in a negative way."
How quickly the cuts go from abstraction to reality might determine whether Congress and the president can find a way to avoid them — or to quickly roll them back.
—Michael D. Shear reported from Washington, and Michael Cooper from New York.