At a time when cuts in federal jobs are being considered on Capitol Hill, a group of lawmakers has said that an existing initiative to limit employment at the government’s largest employer, the Defense Department, threatens to raise costs and allow contractors to take over work that should be performed by federal employees.

A letter from Rep. Maurice Hinchey (D-N.Y.) signed by 130 other House members said that the Pentagon’s policy of limiting the Defense civilian workforce to 2010 levels has “raised concerns that managers could be prevented from using civilian employees even when they cost less or the work is sufficiently sensitive or important that it should be performed by civilian employees . . . it makes no sense to prevent Defense managers from using civilian employees simply because they are civilian employees.”

The letter coincides with a planned vote in the House this week on a Republican-sponsored budget plan that seeks to reduce federal employment by 10 percent through 2015 through a partial hiring freeze. The Obama administration has issued several statements denouncing the planned job losses, including one released Wednesday saying they “would lead to unprecedented cuts in government services.”

The Defense Department would be included in that reduction, although the budget plan envisions the total cut being government-wide, not agency-by-agency. Defense employs more than 700,000 of the 2.1 million executive branch federal workers outside the intelligence agencies and the U.S. Postal Service.

The letter from the House members said that the mix of civilian federal employees and contractors should be based on cost and considerations of whether the work is governmental in nature. “If the Department insists on capping the civilian workforce at [fiscal year] 2010 levels, a similar cap should be applied to the service contract spending levels,” it said.

The American Federation of Government Employees, which represents employees at many Defense facilities, echoed that argument. “This ill-conceived cap has forced managers to cut tens of thousands of federal jobs, while the much larger and more expensive contractor workforce continues to grow unchecked,” AFGE president John Gage said in a statement. “It’s long past time that we restore some balance and fairness to how the entire workforce is managed and treated.”

The letter from the House members follows an announcement by the Air Force that it will offer early retirement and separation incentive payments to certain civilian employees in order to meet the goal of maintaining its civilian staff at 2010 levels, the third recent round of such offers.

Affected employees will receive notices May 1 asking whether they are interested in leaving and typically will have to file an application by May 14. They will need to be off the job by Aug. 31. Even with the incentives to encourage employees to retire or quit voluntarily, the Air Force did not rule out the possibility of layoffs later in the year.

At a Senate hearing Wednesday, Jo Ann Rooney, acting undersecretary for personnel and readiness, said that reductions in the department’s civilian workforce “are being executed very deliberately to minimize adverse impacts” such as “potentially paying more for contracted services or inappropriately realigning work to the private sector.”

In her prepared statement, Rooney said the department is putting place guidance the Obama administration issued last fall regarding what types of work are “inherently governmental” or that otherwise should be reserved for performance by federal employees.

She added that even while holding the workforce to 2010 levels overall, the Pentagon is allowing limited staff increases for work including acquisition, cyber security, intelligence and security guards, and for bringing in-house some previously contracted services.

This story has been updated.