A group of liberal senators, including Sens. Bernie Sanders, D-Vt., and Elizabeth Warren, D-Mass, want the Trump administration not to strip labor rights from civilian Department of Defense employees.

The two — joined by Sens. Edward J. Markey, D-Mass.; Sherrod Brown, D-Ohio; and Kirsten Gillibrand, D-N.Y. — sent a letter to Defense Secretary Mark Esper on Monday urging him to commit not to use the authority granted by President Donald Trump’s Jan. 29 memo to exempt DoD civilians from civil service law. Warren led the letter.

“President Trump’s decision to allow you to bust DoD workers’ unions appears to be motivated purely by an anti-union bias and will harm national security,” wrote the senators. “DoD’s unionized civilian federal workforce provides key support to DoD’s mission and the military forces that carry it out, and stripping away these workers’ rights would result in profound damage to them and to DoD’s national security mission.”

The letter ― the latest Capitol Hill pushback to the move ― came days after Esper declined to commit to using the new authority and said that he did not request it. In congressional testimony, Esper said he was still reviewing the recent presidential decision and that he was not involved in the decision by the White House.

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Trump’s memo cites citing national security concerns and the need for flexibility to adapt to new technologies as it offers Esper the waiver to the 1978 Federal Service Labor-Management Relations Statute.

But the senators questioned the legality of the president delegating this authority and refuted the argument that collective bargaining and labor rights can be incompatible with national security. They argued that unions, which have long existed in national security agencies, strengthen national security by providing stability for workers and ensuring a strong, well-trained, professional workforce.

“The Memo is essentially an anti-union rant, and its argument for exemption is assured without justification or presented the and seems to be generated by an ideological desire to weaken unions rather than a concern with national security,” the letter reads, adding that unions have long existed within national security agencies.

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“The assertion that employees’ collective bargaining rights are ‘incompatible’ with the mission of the DoD, and that civil servants’ ability to advocate for safe, improved working conditions could somehow ‘sacrifice [DoD’s] national security mission’ is erroneous.”