Democrats on the House Armed Services Committee are seeking to bar the Trump administration from detaining any new terror suspects at the prison facility at Guantanamo Bay, Cuba.

The committee’s annual policy bill, according to a copy obtained by Defense One, prohibits the use of funds to detain additional individuals “including United States citizens” and “would prohibit detention of additional individuals under the law of war or pursuant to a military commission proceeding.” It also requires the administration to come up with a plan “identifying a disposition other than continued law of war detention” at Gitmo for each of the 40 detainees currently held there.

Democrats have long sought the closure of the controversial detention facility, created to hold al Qaeda and Taliban fighters in the days following the Sept. 11 attacks under the George W. Bush administration. President Barack Obama sought to shutter it, but was stymied by a Republican Congress, which placed a prohibition on transferring any of the detainees onto U.S. soil.

The legal basis for sending ISIS fighters to the prison remains an open question. While administration officials insist the option is still open, they privately admit that there’s a reason the Trump administration hasn’t tried it yet. Both conservative and liberal legal experts say the government would be on uncertain footing, legally, if it were to try to argue that ISIS fighters qualified as prisoners of war under the 2001 or 2002 congressional authorizations for use of military force passed in the wake of Sept. 11 and still in use today.

But the provision is still likely to face stiff headwinds in the Republican-controlled Senate and the White House. President Trump has vowed to keep the facility open, promising on the campaign trail to fill it up with “bad dudes.” A group of prominent Senate Republicans have called for the administration to send suspected ISIS fighters to the base, which sits on land on permanent lease from Cuba.

Meanwhile, the current detainees are growing older and older. Several recent high-profile medical emergencies at the facility have highlighted the challenges for U.S. military personnel to provide complex care at the remote outpost, perched on the southeast tip of Cuba. In January, the judge in the 9/11 case canceled a pre-trial hearing and then waited 16 hours to be flown to Miami for emergency surgery for a detaching retina.

The chairman’s mark-up of the legislation also includes a nonbinding provision expresses the sense of lawmakers that the “increasing age of detainees… poses challenges for the provision of medical care and that the United States has an ongoing obligation to provide medical care to detainees at Guantanamo that meets appropriate standards of care.” It bans transferring any of the remaining prisoners to Yemen, Somalia, Libya or Syria — but it replaces the blanket prohibition on transferring prisoners to the U.S., meaning that such a transfer would be possible if the text were to become law.

That provision is also likely to struggle in the upper chamber and on Pennsylvania Avenue. But there is some momentum for Gitmo reforms on Capitol Hill: In its version of the bill, the Senate Armed Services Committee for the first time approved a small carve-out in the ban on transferring detainees to the United States, for medical treatments the Defense Department cannot provide on-site.

Senior officers at the base have been petitioning policymakers to give them guidance on how they are expected to care for their aging detainee population, who must be given the same level of care as U.S. service members under the Geneva Conventions.

The United States hasn’t sent a new detainee to Gitmo since 2008. In order to hold a detainee at Guantanamo Bay as a prisoner of war, the U.S. must be able to prove that he or she belongs to a group with whom America is in a state of armed conflict. Both the Obama and Trump administrations have claimed that ISIS is an “associated force” covered by the military authorizations passed by Congress after 9/11. But ISIS formally split from al Qaeda in 2014, and the courts haven’t weighed in on this rationale. Sending an ISIS fighter to Gitmo would almost certainly invite immediate legal challenge — and it’s not at all clear that the administration would prevail, legal analysts say.

The House committee’s legislation is expected to be made public on Monday and marked up on Wednesday.