Forty-five senators, including one Republican, are urging Defense Secretary James Mattis to advise President Trump against implementing a ban on transgender troops he announced this week on Twitter.

“We strongly oppose this policy change and urge you to advise the president against it,” the senators wrote in a letter to Mattis on Friday. “This announcement contradicts existing Defense Department policies, undermines our military readiness and puts our transgender service members as well as their commanders in an impossible situation.”