President Donald Trump tweeted on Wednesday morning thattransgender individuals will no longer be able to serve in the U.S. military “in any capacity.” Some legislators are already working on overturning that decision.

Seeing as the Pentagon lifted the ban on transgender people in the U.S. military last year and Congress has been heavily focused this week on debating an Obamacare repeal, this announcement was completely unexpected.

Despite the suddenness of the decision, members of Congress have acted quickly to effect change. Politico reporter Connor O’Brien announced via Twitter that Rep. Scott Peters (D-Calif.) filed an amendment to the Department of Defense Appropriations bill to overturn Trump’s ban on transgender people serving in the military.

The amendment stipulates that the government cannot use funds allocated for defense to “implement, enforce, or observe in any way any directive from the President of the United States that bars or restricts the ability of transgender persons to serve in the Armed Forces.”

In a statement regarding the amendment and why he was driven to file it, Peters said he couldn’t “think of anything more backwards than turning away a brave, qualified American who wants to serve in our all-volunteer military because of their gender identity.”

“There are already thousands of transgender Americans serving honorably and openly in our Armed Forces. There is a former member of SEAL Team 6 – the most elite military unit in history – that came out as transgender,” the statement says.

“Just as we did with the repeal of Don’t Ask, Don’t Tell, Congress must stand up against backwards politics, trust our military leaders, and put national security first.”

Additionally, Sen. Kirsten Gillibrand (D-N.Y.) released a statement calling Trump’s directive “harmful” and “misguided.”

“I will introduce legislation and will fight to overturn this discriminatory decision,” she said.

Many other members of Congress from both sides have spoken out against the transgender ban ? including Sen. John McCain (R-Ariz.). McCain released a statement saying the Senate Armed Services Committee, which he chairs, “will continue to follow closely and conduct oversight on the issue of transgender individuals serving in the military.”