WASHINGTON– President Donald Trump said on Wednesday he would ban transgender people from the U.S. military, a move appealing to some in his conservative political base but creating uncertainty about the fate of thousands of transgender service members.

The surprise announcement by Trump, who as a presidential candidate last year vowed to fight for lesbians, gays, bisexuals and transgender people, came in a series of morning Twitter posts. It drew condemnation from rights groups and some lawmakers in both parties as politically motivated discrimination but was praised by conservative activists and some Republicans.

The administration has not determined whether transgender individuals already serving in the military would be immediately thrown out, a point the White House and Pentagon have yet to decide, Trump spokeswoman Sarah Sanders said.

A transgender ban would reverse Democratic former President Barack Obama’s policy and halts years of efforts to eliminate barriers to military service based on sexual orientation or gender identity.

“After consultation with my Generals and military experts, please be advised that the United States Government will not accept or allow transgender individuals to serve in any capacity in the U.S. Military,” Trump tweeted, without naming any of the generals or experts.

“Our military must be focused on decisive and overwhelming victory and cannot be burdened with the tremendous medical costs and disruption that transgender in the military would entail,” he said.

“This was about military readiness,” Sanders told a briefing. “This was about unit cohesion. This was about resources within the military, and nothing more.”

Critics said the health costs of caring for transgender service members were a tiny portion of the military’s healthcare budget and Trump’s policy change was based on prejudice.

His action unleashed a torrent of legal threats from civil liberties advocates seeking plaintiffs willing to challenge the ban in court and sparked a protest by hundreds who rallied outside an armed forces recruiting station in New York.