Federal employee unions, having guided their members through government shutdown threats in 2011 and 2012, are arming for another possible shutdown in 11 days if Congress cannot resolve its partisan fiscal battle, taking their opposition to Congress and the Obama administration.

The International Federation of Professional & Technical Engineers, which represents 25,000 federal workers, mostly military civilians, appealed Thursday to Office of Management and Budget Director Sylvia Burwell to ensure that government workers are compensated for lost pay if federal agencies shut down on Oct. 1, the first day of the new fiscal year.

“IFPTE insists that our nation’s federal employees are made whole when they eventually return back to work,” union President Gregory Junemann told Burwell in a one-page letter.

“IFPTE asks you, as the head of OMB and as someone who will no doubt play a key role in any negotiation aimed at re-opening the government, to INSIST that federal employees get paid for all days missed during a government shutdown,” Junemann wrote.

“We urge the Obama Administration not to use federal worker back pay as a bargaining chip to extract something in return. Rather we ask that it be made clear from the very start of any negotiation that government workers will get every penny owed to them when the government reopens,” his letter said.

Junemann said that the financial hardship on federal workers during the current round of partisan bickering on Capitol Hill over government spending and the president’s health-care law is particularly dire now. Feds are in their third year of a pay freeze, and almost half the workforce has lost several days of pay to furloughs since March because of the automatic budget cuts known as sequestration.

The union’s plea comes two days after Burwell issued a memo to federal agencies to start planning for a partial shutdown on Oct. 1 if Congress does not reach an agreement to fund the government before then. In a partial shutdown, some crucial operations continue, and employees that are needed to keep them going keep coming to work.

After two shutdowns in the 1990s, Congress awarded back pay to federal employees who were out of work.

Also this week, the American Federation of Government Employees, the largest employee union, exhorted its members to “demand that lawmakers stop the sequester and stop threatening to shut down the government if they don’t get their way.”

“Send a message to your lawmakers and tell them to stop playing political games,” AFGE President J. David Cox told union members in an e-mail. Referring to House Republicans’ decision to vote Friday on a measure to keep the government running and defund the president’s health-care law, Cox said, “Over a dozen Senators and eighty  Representatives are pushing for a scenario that should be unthinkable: they want to use a government shutdown as leverage to enact an extreme, right-wing agenda that includes continuing the Sequester for the next nine years. And these folks are picking up more support every day.”