Reacting to the horrifying mass shooting in Orlando on Sunday, President Obama predictably called for more gun control, but he was predictably vague:

Today marks the most deadly shooting in American history. The shooter was apparently armed with a handgun and a powerful assault rifle. This massacre is therefore a further reminder of how easy it is for someone to get their hands on a weapon that lets them shoot people in a school, or in a house of worship, or a movie theater, or in a nightclub. And we have to decide if that's the kind of country we want to be. And to actively do nothing is a decision as well.

As is usually the case with mass shooters, Omar Mateen, the man who attacked the gay night club Pulse early Sunday morning, did not have a criminal or psychiatric record that would have legally disqualified him from owning firearms. We know that because he purchased his guns legally from one or more federally licensed dealers, passing a background check each time, and had been employed since 2007 by the security company G4S, which would have run its own record check. Hence "universal background checks," Obama's favorite gun control proposal, manifestly would not have made a difference in this case. Nor is it likely that the restrictions imposed by a new federal "assault weapon" ban would have reduced the death toll, since they have little to do with a gun's lethality and in any case would leave millions of the targeted firearms and "high capacity" magazines in circulation.

The one Obama-supported gun control that might have posed an obstacle for Mateen is a policy of prohibiting anyone on a federal "watch list" from buying firearms. The New York Times reports that Mateen, who was twice investigated by the FBI because of suspected ties to terrorism, "is believed to be on at least one watch list." Assuming that's true, a law that made inclusion on a watch list a disqualifying criterion for gun purchases, either automatically or at the discretion of the attorney general, could have prevented Mateen from buying guns at a federally licensed store. If the background check requirement had been extended to all gun transfers, it would have been illegal for Mateen to buy a gun privately as well—although that does not mean it would have been impossible, since it's not clear how the feds could possibly enforce such a mandate.

Both of the major-party candidates for president seem to agree with Obama that people on watch lists should not be allowed to buy guns. "If somebody is on a watch list and an enemy of state and we know it's an enemy of state, I would keep them away, absolutely," Donald Trump told ABC News last November. "We have to up our game against terrorists abroad and at home, and we have to take account of the fact that our gun laws and the easy access to those guns by people who shouldn't get them," Hillary Clinton said on ABC's This Week in December, complaining that Congress "refuse[s] to prohibit people on the no-fly list from getting guns." She dismissed concerns that innocent people could lose their Second Amendment rights based on mere suspicion, saying, "We have a list. If you are on that list and you believe you should not be on that list, we have a process to actually raise your objections about being on that list."

The problem is that it's easy to get on a watch list and hard to get off. The FBI's so-called Terrorist Watchlist, which is supposedly limited to individuals "reasonably suspected of being involved in terrorist activity," may include more than 1 million people, perhaps two-fifths of whom have "no recognized terrorist group affiliation." The Denying Firearms and Explosives to Dangerous Terrorists Act, which the Senate rejected in December, would have given the attorney general essentially unlimited power to block gun sales, provided he "determines" that the buyers are "appropriately suspected" of involvement in terrorism and "has a reasonable belief" that they "may" use the firearms "in connection with terrorism." The bill did not even require that a blocked buyer appear on a watch list, and the criteria for upholding the attorney general's decision against a buyer's appeal were highly permissive: The government would merely have to show it was more likely than not that the attorney general had met the statutory criteria, meaning a low probability that someone is connected to terrorism would suffice to take away his Second Amendment rights.

Even in Mateen's case, there was little evidence that he was planning to carry out an attack. The Times says the FBI investigated him in 2013 "after reports from Mr. Mateen's co-workers that he, the American-born son of Afghan immigrants, had suggested he may have had terrorist ties." Nothing came of it: "The F.B.I. interviewed him twice, but after surveillance, records checks and witness interviews, agents were unable to verify any terrorist links and closed their investigation." The following year, "the F.B.I. discovered a possible tie between Mr. Mateen and Moner Mohammad Abusalha, who had grown up in nearby Vero Beach and then became the first American suicide bomber in Syria, where he fought with the Nusra Front, a Qaeda-aligned militant group." But "the F.B.I. closed its inquiry after finding 'minimal' contact between the two men."

Assuming Mateen nevertheless remained on the FBI's watch list (as the Times suggests), that means someone who was twice cleared of involvement in terrorism—someone who may have attracted attention based on nothing more than misconstrued comments and a passing acquaintance with a future suicide bomber—would nevertheless be deemed suspicious enough to lose his Second Amendment rights if Obama, Clinton, and Trump had their way. In retrospect, it is easy to say Mateen should not have been allowed to buy guns. But almost none of the people who face similar suspicions based on similarly meager evidence end up doing anything like what Mateen did. To block gun purchases by someone like Mateen, the net must be cast wide enough to ensnare lots of innocent people, who will lose their constitutional rights without anything resembling due process.