As the clock struck midnight Monday (0400 GMT), House Republicans were demanding that the Senate negotiate their demand for a one-year delay in making millions of people buy health insurance under President Barack Obama's 2010 health care law.
Minutes before midnight, the White Houseordered a shutdown.
The Democratic Senate on Monday twice rejected Republican demands to delay key portions of what has become known as "Obamacare" as a condition for keeping the government open.
An estimated 800,000 federal workers faced furloughs. Critical functions like air traffic control and military operations will continue. Social Security benefits will be paid. National parks and most federal offices will close.