Overnight, Democrats held the Senate floor to force a vote on two gun safety laws aimed at preventing another tragedy like the murder of 49 people at a gay club in Orlando. It was long hoped-for political surge to limit the availability of powerful weapons used in numerous mass shootings in recent years.

But though a measure to expand background checks to gun sales made on the internet and at gun shows is overdue, another that would link gun sales to the so-called “terror watch list” is troubling. Not only is the list poorly defined and maintained, but it is also under intense legal challenge and likely unconstitutional.

The immediacy of the fix is appealing: Omar Mateen, the killer in Orlando, had been on a terror watch list, though he was taken off it before he purchased the weapons used in the killings. On its face, it seems insane that the US maintains a list of people who can’t fly on planes, but who can buy military weapons.

For this reason, everyone from president Barack Obama to Donald Trump want to keep people on that watch list out of gun shops.

But not too long ago, it seemed crazy that the US was keeping a list of citizens who it could not charge with a crime but it could bar from flights, with limited avenues of appeal—a bit of post-9/11 panic that stood until the American Civil Liberties Union (ACLU) filed a lawsuit against the government in 2010 on behalf of several people who say they are blocked from flying for no good reason. (The no-fly list is a sub-set of the government’s Terrorist Screening Database.)

In 2014, the case led to the declaration that the no-fly list is unconstitutional, and the government began offering limited redress to people on the list. But people challenging their status could still not see the evidence or examine the witnesses against them. In December 2015, the ACLU went to court again, and a judge ruled the government’s changes were acceptable in general, but still hasn’t ruled on the individuals who brought the case.

“We continue to believe, despite the court’s ruling, that the revised redress process doesn’t meet the standard of due process,” ACLU staff attorney Hugh Handeyside says. The ACLU has yet to take a position on the gun safety amendments currently under discussion.

Following the San Bernadino attack, when there were similar calls to bar people on the terror watch list from buying guns, the ACLU condemned the “predictive judgments” about what people might do in the future that watch lists rely upon. “There is no constitutional bar to reasonable regulation of guns, and the No Fly List could serve as one tool for it,” Hina Shamsi, director of the ACLU’s National Security Project, wrote, “but only with major reform.”

Today, there are at least 250,000 people on the terrorist watch list, though we don’t know how many in total. Law enforcement can add people to the list based on “reasonable suspicion,” and there is no process to challenge your placement on the list. Most people who are on the list don’t know they’re on it until they are prevented from boarding an aircraft or told by a law enforcement officer. It is an investigative tool that, when used as a legal judgement, appears ripe for abuse, bias, and profiling.

Those that think we should simply ban weapons like the AR-15 from civilian use will probably not be dismayed if a chunk of the populace is prevented from buying them, even under arbitrary and capricious rules. It may seem easier to let a secret court assess who should have these military weapons than to convince society to ban them entirely. But rarely do such measures come without unintended consequences. And these proposed policies leverage the same politics of fear that many in the US came to regret after 9/11.

After the San Bernardino killings, both of these amendments came up for a vote, and failed. It’s possible we could see the “terror gap” amendment pass today, though the gun lobby is lukewarm on the provision, saying that the terror watch list could be used to prevent gun sales but only with significant reform. But we can expect that the list it is based on will be repeatedly challenged in court. It’s even possible that putting the gun lobby’s clout on the same side as civil liberties groups may hasten the demise of the watch list approach.

If gun safety advocates want to ban the sale of military weapons to civilians, the terrorist watch list may be a tempting place to start, but its foundation is not just ugly; it’s also unstable.