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.”

The letter was organized by Sen. Kirsten Gillibrand Kirsten GillibrandSunday shows preview: Justice Ginsburg dies, sparking partisan battle over vacancy before election Suburban moms are going to decide the 2020 election Jon Stewart urges Congress to help veterans exposed to burn pits MORE (D-N.Y.), who has promised to introduce legislation blocking the president from implementing the ban.

All signatories are Democrats except for one — moderate Republican Sen. Susan Collins Susan Margaret CollinsGraham: GOP will confirm Trump's Supreme Court nominee before the election Gardner signals support for taking up Supreme Court nominee this year Tumultuous court battle upends fight for Senate MORE (Maine).

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On Wednesday, Trump announced on Twitter that he would ban transgender people from serving in the military.

A day later, Chairman of the Joint Chiefs of Staff Gen. Joseph Dunford advised the military that the Pentagon will not take any action until Trump sends over an official directive.

In their letter, the senators said they appreciate Dunford’s announcement.

But, they added, Mattis should also “at a minimum” make sure no one is discharged until the six-month policy review he originally promised is complete.

At the end of June, Mattis said he would undertake a review of the Pentagon’s transgender policy after he delayed allowing transgender recruits to enlist for six months.

“We further write to request that, at a minimum, you do not separate any service member due to the person’s gender identity until you have completed the assessment that you announced on June 30, have reported back to Congress about any challenges that you foresee in the accession and retention of transgender troops, and determined the department is unable to mitigate these challenges,” the senators wrote.

The senators added that there have been no reports of issues since June 2016, when transgender troops were allowed to begin serving openly.

Kicking them out of the military, the senators argued, would be “cruel and discriminatory,” harm readiness by requiring the military to replace them and hurt morale by having their colleagues see them discharged for their identities.

“Any American who wants to serve and meets the standards should be allowed to serve our country,” they wrote. “Transgender service members are serving with honor and distinction today and we ask that you, as our secretary of Defense, assure them that their service will not be ended simply because of who they are.”