WASHINGTON -- The Obama administration told union leaders Thursday morning that federal employees will know by the end of the week whether they can report to work Tuesday if the government shuts down.

Supervisors will tell employees informally by the end of the day Friday if they can expect to be furloughed, the president of the National Treasury Employees Union said in a statement. They would receive an official notice on Tuesday if a continuing resolution to fund the government is not passed in Congress and signed into law by then.

"They're very, very worried," said J. David Cox, the president of the American Federation of Government Employees, which represents 650,000 federal workers. "Most members make $35,000 to $40,000 a year. That's not of a lot of money. Let's be real, they're living paycheck to paycheck."

Union presidents were briefed by the Office of Management and Budget and by the Office of Personnel Management, which said agencies' shutdown plans will be online by Friday afternoon.

Plans are expected to be similar to the ones made in 2011 in preparation for a shutdown that was narrowly avoided. They outline how many workers are deemed "excepted" because they are considered essential to performing activities required by law or for the protection of life and property. Those who are not paid with annual appropriations would also avoid furloughs.

If those plans are an indication of who would stay on the job come Tuesday, United States Forest Service firefighters would keep monitoring the Rim fire that is still smoldering in California. Three-quarters of employees would also report to work at the Federal Emergency Management Agency, which has been opening disaster centers in Colorado to provide flood relief.

But those who faced furloughs under the 2011 plan included more than 80% of the National Park Service's 20,000 employees. In 1996, the last time the government shut down, parks and monuments across the country were closed, deterring an estimated 9 million visitors.

More than 90% of the Smithsonian's 6,400 workers would be furloughed, spokeswoman Linda St. Thomas said Tuesday, and all its facilities, including museums and the National Zoo, would close.

Union officials said a shutdown would hurt federal employees who have already endured a three-year pay freeze and about a week's worth of furlough days this year.

Cox said officials expected that employees required to work would still be paid, but not necessarily on time.

In the past, Congress has approved back pay for workers furloughed during a shutdown, but there is no guarantee that would happen this time, Cox said.

The threat of a shutdown has "created anxiety and uncertainty" among workers, Colleen M. Kelley, the treasury union president, said in her statement.

"Federal agencies have had to devote time and resources to develop yet another crisis plan, distracting agencies from their critical missions," she said in the statement. "And, if the government shuts down, the public will be further harmed by the loss of vital services people need and depend upon."

Cox said the administration told the unions that President Obama was "committed to federal employees" and is still pursuing a budget that has a 1% pay increase for workers.

"Our members have lived in constant uncertainty this entire year," Cox said. "We've been in furloughs, we've been in pay freezes. We've been in constant trouble all year long.

"Our members just want to go to work. They want to go to work, they want to provide services to the American people."

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becca.clemons@latimes.com