The Capitol is seen on a gloomy Monday in Washington, Sept. 23, 2013. (J. Scott Applewhite/AP)

A government shutdown next week would jeopardize the paychecks of more than 800,000 federal workers who could be told to stay home. More than 2 million other employees who are deemed essential by the government — including the active military — would be entitled to their salaries but might not get paid on time.

While there is no law requiring that nonessential employees be compensated if they are ordered off the job, Congress has in the past voted to reimburse their losses once shutdowns ended.

But this go-round could be different. The bitterly divided Congress includes many lawmakers who are unsympathetic to the plight of federal workers and could be loath to help them recoup their money.

“It’s a very different time and a very different Congress,” said Colleen Kelley, president of the National Treasury Employees Union, which represents 150,000 federal workers. “I’m concerned when employees who were here remember that last time employees were paid and think it will happen again, because it’s not a given at all.”

After the past two shutdowns in the 1990s — when federal workers were furloughed for five days in November 1995 and 21 days from December of that year into January — Congress passed a bill awarding them back pay.

Congressional reporter Ed O'Keefe outlines the key players in this week's showdown between the House and Senate over the continuing resolution to fund the government. (The Washington Post)

(Federal workers: How do you feel about another potential shutdown? What is your agency doing to prepare?)

Even if Congress fails to resolve its budget standoff by the deadline Monday and much of the government runs out of money, civilian and military personnel who are considered vital to national security, public safety, health and welfare would be called to work — about 1.3 million civilian employees and 1.4 million in the military.

By law, they all must be paid. But the law does not say when.

If a shutdown ends within their two-week pay cycle, their checks still would be delivered on time. If it drags on longer than two weeks, they would work without being paid for a time.

All active-duty members of the military, for instance, are paid on the 1st and 15th of the month. Even if there is a shutdown, they would be paid Oct. 1 for the previous two weeks of work, but the following paycheck would be at risk if the government closure is prolonged.

If a shutdown lasts just a few days, it probably would not affect paychecks that are scheduled for mid-October. But even the government’s heavily automated payroll system needs several days’ notice to set its gears in motion, so if a shutdown ended a day or two before paychecks at a given agency are due, those employees deemed “essential” could have their pay delayed, several union and government officials said.

Compared with the shutdowns of the 1990s, many more federal workers are in danger of being furloughed this year, because Congress has not passed a single funding bill. In the past, Congress had passed appropriations bills that funded various large agencies, including the Defense Department, meaning they could continue to operate even if other parts of the government could not.

Federal agencies have started bracing for a shutdown by digging out their plans from 2011, when the government almost ran out of money amid fiscal battles between Republicans and Democrats on Capitol Hill.

On Monday, agencies began reviewing which civilians would be told to come to work during a shutdown and which would be ordered to stay home. Defense Department spokesman George Little said overseas operations, including those in Afghanistan, would not be directly affected.

“The President and the Secretary know that the uncertainty of the current situation puts our civilian workforce in a difficult situation and, should a lapse occur, it could impose hardships on many employees and disrupt important national security projects,” Deputy Defense Secretary Ashton Carter wrote Monday in a memo to Defense Department employees.

Attached was a lengthy series of questions and answers prepared by the Office of Personnel Management in 2011 about how pay, vacation, health insurance and other benefits would be affected during a shutdown.

Steven Posner, a spokesman for the Office of Management and Budget, said in a statement Monday night that there is “enough time for Congress to prevent a lapse in appropriations, and the administration is willing to work with Congress” to enact a short-term spending plan to fund critical government operations while lawmakers work on a budget for fiscal 2014.

But if there’s no temporary budget by late this week, federal workers will get an e-mail or phone call from their supervisors by Monday telling them to report for work or remain at home.

Those who would not be working would get a few hours Tuesday morning to come into the office to secure their files, send e-mails and put things in order before signing off. It would be illegal for them to conduct any work until they were called back to their jobs.

Almost half of the federal workforce has already been forced to take multiple unpaid furlough days this year, the result of an earlier Washington showdown that set into motion $85 billion in automatic budget cuts known as sequestration. Federal employees are also in the third year of a pay freeze.

“We think we have a very strong case to make that we are total victims of the standoff in Congress,” said Jacqueline Simon, public policy director at the American Federation of Government Employees, the largest federal employee union. She said the union plans a “huge grass-roots political mobilization” to fight for back pay if there is a shutdown.

“Every member of Congress will be hearing from their constituents who are federal employees,” she said.

Some political observers suggested that conservative Republicans in Congress are unlikely to side with the unions in their fight.

“If there’s savings to be achieved by not paying [the federal employees] and we’ve got a conservative Congress that says, ‘They didn’t work. Why would we pay them?’ . . . it’s a lot more problematic as to whether precedent would apply this time around,” said G. William Hoag­land, a former Republican staff director of the Senate Budget Committee who is now senior vice president at the Bipartisan Policy Center.

Rep. Gerald E. Connolly (D-Va.), who has tens of thousands of federal workers in his district, said: “Unfortunately, the ideology of the House majority is rabidly anti-federal-employee. I think it’s going to be very tough sledding to persuade sufficient numbers of them to keep their commitment to federal workers who will find themselves victims” in a shutdown.

A House Republican aide said many conservatives would have a hard time justifying “making them whole for work they didn’t do.” The aide, who was not authorized to speak publicly on the shutdown issue and asked not to be identified, expected that the issue of back pay would be resolved with talks on sequestration and the debt limit, another looming issue.

Rep. Jason Chaffetz (R-Utah), who calls himself a tea party Republican and voted for the House bill tying continued government spending to the defunding of the Affordable Care Act, said it is a shame that rank-and-file federal employees are caught up in a fight over President Obama’s health-care law. “I rail against the federal government,” Chaffetz said, “but we have a lot of patriotic federal employees who should not be in the middle of this skirmish.”

The president and political appointees are exempt from furloughs, although some White House employees are considered essential while others are not. Members of Congress also would keep working, deciding which of their staff members to send home and which to keep at the office.

For millions of federal retirees who may be concerned about their annuity checks, which arrive on the first day of each month, the Office of Personnel Management said they would be paid on Oct. 1 as normal.