Washington-area lawmakers of both parties also are sponsoring legislation ensuring that all federal employees — essential and nonessential — would be paid following any shutdown.

Reps. James P. Moran Jr. (D-Va.), Gerald E. Connolly (D-Va.) and Frank R. Wolf (R-Va.) — who represent three of the most fed-centric congressional districts in the nation — are backing the “Federal Employee Retroactive Pay Fairness Act.” Sens. Barbara A. Mikulski (D-Md.), Benjamin Cardin (D-Md.), James Webb (D-Va.) and Mark Warner (D-Va.) — senators for the two states bordering Washington — are co-sponsoring similar legislation in the Senate.

Federal workers “should not be scapegoats for the decisions of politicians,” Mikulski said.

According to recent U.S. Census data, the D.C. Metro area — which includes suburbs in Maryland and Northern Virginia — has about 345,000 full-time civilian federal workers, the largest concentration of them in the country. The figures do not include private government contractors.

About 85 percent of federal employees live outside the Washington area. After the nation’s capital, the cities with the highest percentage of their workers as full-time feds are Virginia-Beach/Norfolk, Baltimore, Oklahoma City, San Diego, San Antonio, Jacksonville, Kansas City, Mo., Richmond and New Orleans, according to Census figures.

The Obama administration is being sued by the largest federal labor organization, which says it is illegal to make essential government employees work when money has not been appropriated to pay them.

In a lawsuit filed in the U.S. District Court for the District of Columbia, the American Federation of Government Employees argues that the administration’s plans to have most employees continue working during a shutdown violates the Constitution’s Appropriations Clause and the Thirteenth Amendment, which abolished slavery.

“Hundreds of thousands of federal employees will be required to work during a shutdown, and there’s no guarantee that Congress will keep the administration’s promise to pay those employees once the shutdown is over,” said AFGE President John Gage.

Administration officials have said that employees needed to protect life and property who work during a shutdown would be paid retroactively once Congress and the president approve a spending measure. Nonessential personnel also go unpaid during a shutdown, and the Obama administration said this week it supports having furloughed workers paid after a shutdown.

“The Constitution requires an appropriation by Congress before federal dollars can be spent, no exceptions,” Gage said. “Without an appropriation, the agencies simply can’t spend money or incur debts by forcing employees to work.”

Staff researcher Dan Keating contributed to this report.