The Office of Personnel Management said federal agencies in the Washington area would be closed Thursday, meaning only emergency employees would be expected to work.
The US House of Representatives also decided to shut down on Thursday so members could leave Washington and avoid the storm if they wished.
The Senate decided to wait before taking similar action.
"We're watching Isabel to see how it develops," said Republican Senator Mitch McConnell.
Senate committee hearings scheduled for Thursday were still on the calendar late Wednesday.
Three mid-Atlantic coastal states -- North Carolina, Maryland and Virginia -- declared emergencies Wednesday and thousands of residents fled the coast to safety inland.
Washington, nestled between Virginia and Maryland about 100 mileskilometers) from the coast, was expected to be hard hit as the storm moves inland.
The National Weather Service said the hurricane, packing winds of 110 miles (180 kilometers) an hour, was moving toward the North Carolina coast at about nine miles (15 kilometers) an hour.