The justices, voting 5-4, put on hold lower court decisions that had blocked the administration’s planned ban from taking effect. Justices Ruth Bader Ginsburg, Stephen Breyer, Sonia Sotomayor and Elena Kagan dissented.

The court stopped short of agreeing to hear arguments on an expedited basis, as the administration sought. But by letting the ban take effect, the court gave the administration a major victory and hinted the justices ultimately will uphold the restrictions.

Transgender troops have been serving openly since June 2016, when President Barack Obama’s administration began lifting a longstanding prohibition. Opponents of the ban say reinstating it would violate the Constitution’s equal protection clause.

“This case is about whether men and women who want to serve in the United States Armed Forces to protect their country and who are able and otherwise qualified to do so should be barred from military service because they are transgender,” according to court filings on behalf of current and prospective military members.

The administration said courts should defer to the Defense Department’s judgment. A 2018 Pentagon report said that having transgender troops “risks unnecessarily adding to the challenges faced by leaders at all levels, potentially fraying unit cohesion, and threatening good order and discipline.”

Then-Defense Secretary James Mattis commissioned the study after Trump said in a July 2017 tweet that the government would “not accept or allow transgender individuals to serve in any capacity in the U.S. Military.” Mattis said he wasn’t consulted before the tweet. He left the Trump administration at the end of 2018.

The administration’s policy is less absolute than Trump’s original tweet suggested. It lets people continue to serve if they began transitioning their gender in reliance on the Obama policy. But it would bar anyone from starting gender transition while in the armed forces. And the policy would block people from joining the military if they have already transitioned.

Estimates vary of how many transgender people serve in the military. A 2016 Rand study estimated that between 1,320 and 6,630 transgender people were on active duty. The Mattis report said that since June 2016, about 900 people on active duty have been diagnosed with gender dysphoria — the medical term for a conflict between one’s physical gender and the gender with which the person identifies. The report also said that a 2016 survey of the military found that almost 9,000 people identified as transgender.

The administration asked the court to take the unusual step of bypassing the federal appeals court level and directly deciding whether trial judges were right to block the ban.

U.S. Solicitor General Noel Francisco told the justices the issue was one of “imperative public importance,” thus warranting special fast-track consideration. He said immediate review was needed “to ensure that the injunction does not remain in place any longer than is necessary.”

But critics of the policy said the administration was improperly rushing the issue.

“Openly transgender persons have been serving in the military for two and a half years and leaders from each branch have testified that their service has not harmed the military,” Washington Attorney General Bob Ferguson argued in court papers. “Although the government cries that the sky is falling, it has offered no evidence to support its dire claims.”