A bipartisan group of senators on Thursday asked President Trump to “reconsider” his recent memo granting Defense Secretary Mark Esper the power to end collective bargaining for the Pentagon’s 700,000-person civilian workforce, describing the measure as a misuse of federal labor law.

Last month, Trump quietly signed a memo delegating to the Defense secretary his legal authority to exempt portions of agencies from the law guaranteeing federal employees the right to unionize, arguing such a decision may be necessary because of the need for “flexibility.”

“When new missions emerge or existing ones evolve, the Department of Defense requires maximum flexibility to respond to threats to carry out its mission of protecting the American people,” Trump wrote. “This flexibility requires that military and civilian leadership manage their organizations to cultivate a lethal, agile force adaptive to new technologies and posture changes. Where collective bargaining is incompatible with these organizations’ missions, the Department of Defense should not be forced to sacrifice its national security mission and, instead, seek relief through third parties and administrative fora.”

Since the memo became public, it has received a cool reception from labor groups and lawmakers. Federal employee unions like the American Federation of Government Employees decried the decision, and Esper faced pointed questions on the issue from House Democrats Wednesday.

In a letter to Trump Thursday, Sens. Susan Collins, R-Maine; Mark Warner, D-Va.; Ben Cardin, D-Md.; Chris Van Hollen, D-Md.; Tim Kaine, D-Va.; and Gary Peters, D-Mich., said good labor-management relations are critical to the Defense Department’s effectiveness.

“We all agree that the Department of Defense requires flexibility to respond to the challenges that our nation faces,” they wrote. “However, collective bargaining is not only compatible with this needed flexibility, but also is a key component in preserving flexibility by giving employees a voice in the system and providing avenues for management to receive feedback.”

The senators said that Congress intended the provision of federal labor law allowing the president to exempt portions of the federal workforce from collective bargaining to be used in rare, isolated cases, not as part of a potentially department-wide edict.

“Any exemptions permitted by the process are not meant to be given widely to an entire department as a sweeping declaration, but to be carefully considered,” they wrote. “In fact, because of existing safeguards, no president has found it necessary to issue a blanket exemption of all Department of Defense employees from collective bargaining since enactment of the Civil Service Reform Act of 1978. Instead, previous use of this authority has been as narrowly crafted as possible.”

In testimony before the House Armed Services Committee Wednesday, Esper said that he did not request the authority, but declined to provide details about if and how he might employ his new power. He said he would wait until his staff provided analysis and recommendations before taking action.