The idea that government should prevent suspected terrorists from being able to buy guns seemed so obvious in the wake of the Orlando massacre that Donald Trump and Senate Democrats were in agreement about it.

But as Democrats have seen as they’ve tried to turn that idea into a bill that could get 60 votes in the Senate, what seems obvious in theory is a lot more complicated in practice. And the most bipartisan ideas might not be the best ones.

The "terror gap" debate has run right into the debate over surveillance and the war on terror — and splintered like a log in a wood chipper. Suddenly, liberal opinion-makers from Kevin Drum to Trevor Noah are against the Democrats’ proposal.

The reason is simple: If you give the government more power to ban terrorists from having guns, you’re reinforcing the power it has to define who counts as a terrorist.

That power is something of which both many liberals and many conservatives are deeply skeptical. They both worry that the government’s choices about who to target for surveillance and enforcement are unfair — even discriminatory. But they’re worried about discrimination against different populations — and that shapes their objections, and the alternatives they propose.

Using terrorist watch lists and FBI investigations to deny guns gives them more power

Whenever the war on terror meets gun control politics, the terms of the debate shift: Democrats and gun control advocates tend to believe that the threat is dire enough that it’s worth a little collateral damage to civil liberties, while Republicans don’t. In part, that’s because people who support gun control aren’t exactly worried about gun control policies being applied overbroadly: If the proposal keeps more guns off the streets than it’s supposed to, that suits them just fine.

What you need to remember, though, is that this isn’t a separate system getting created for gun control — it’s giving more power to the existing surveillance system. That surveillance system has shifted over time depending on what it’s being used for, and giving it the power to ban gun sales would probably lead it to evolve again.

The current Democratic gun proposal, for instance, would subject two categories of people to higher scrutiny: people on the watch list of "known or suspected terrorists," and people who have been investigated by the FBI in the last five years.

How the FBI conducts terrorism investigations has been hotly debated. And the history of watch lists show that they’re shaped at least as much by political pressures as by intelligence needs. So it's worth wondering how those political pressures might work if those tactics suddenly become the government's most powerful tool for preventing someone from owning a gun.

We have no idea who would be targeted

We have no idea who would actually get flagged for greater scrutiny when they applied for a gun. That’s because we have no idea who is actually on the "terrorism watch list." There’s absolutely no way for someone to find out if she's on it or not.

The ACLU has managed to win a little transparency when it comes to the no-fly list — one of the many lists drawn from the Terrorist Screening Database (TSDB), the list of 800,000 "known or suspected terrorists" that’s typically referred to as the master "terrorism watch list."

When the ACLU sued on behalf of 10 people who believed they were on the no-fly list and demanded to be told why — some of whom were actually stuck outside the US — the government attempted to argue that whether or not they were actually on the list counted as a "state secret."

Despite the victories on the no-fly list, though, "the government refuses to confirm or deny whether people are actually on the master watch list," says Hina Shamsi of the ACLU.

Without that basic starting point, it's impossible to know how many people have been watch-listed by mistake — or might have been legitimately watch-listed to begin with, but might be okay to remove now.

And since the terrorism records that are checked when someone goes through a gun-purchase background check are drawn from that master watch list, that means there’s no way of knowing who would be scrutinized for trying to buy a gun. Ironically, if the Democrats succeed in passing their "terror gap" proposals, attempting to buy a gun and being denied might be the only way for someone to find out for sure if he or she is being watched.

It wouldn’t be a confirmation that the person in question is currently on a particular watch list: Under the proposal, people who have been under investigation at any time in the last five years for terrorism are placed under scrutiny, even if they’re not on the list. And the government doesn't appear to be scrupulously committed to pruning down the list itself. Being acquitted of terrorism charges or having those charges dismissed is actually cited as a reason to put someone on the watch list — not a reason to take him off.

In other words, the proposal is guaranteed to sweep up people who the government has already determined, for sure, don’t pose a threat — and return them to scrutiny all over again.

Due process protections render the policy unworkable

In general, Republicans have gotten a little more skeptical of the government's national-security powers (and Democrats have gotten less so) now that the person wielding those powers is a Democrat. But their responses to Democrats’ gun proposals have shown on whose behalf, in particular, they’re concerned: law-abiding gun owners without any connection to "real" (i.e., foreign) terrorist activity.

For that reason, Republican compromise proposals have focused on adding more checks on who is prevented from buying a gun based on the watch list — not on adding checks to the watch list itself.

One proposal, supported by Sen. John Cornyn (R-TX) and the NRA, would require a judge to decide within 72 hours if the watch-listed person should have his gun background check denied. Another, supported by Sen. Pat Toomey (R-PA), would create a separate list of gun-banned terrorists, which would have to be approved by the secret FISA surveillance court.

Democrats have rejected both of those proposals as totally impractical. As Sen. Joe Manchin (D-WV) put it Thursday: "Due process is what’s killing us right now."

They’re probably right. It would probably be prohibitively difficult for the federal government to suddenly create a fast, standardized process related to the terrorism watch list — because, as far as we know, nothing like that is even close to existing right now.

But that signals bigger problems with the current state of national-security policies. Toomey’s proposal, for example, would add work to an already overworked and secretive FISA court — but the underlying problem is that the FISA court is already overworked and secretive.

This will create pressure to surveil more people — and perhaps encourage more plots

If the terrorism "watch list" and FBI investigations become important criteria for whether or not the government can stop someone from buying a gun, that’s going to shape who gets placed on the watch list and investigated by the FBI.

When it comes to surveillance, though, the problem with the status quo isn’t that the government is getting too little information. It’s that it’s getting too much without being able to figure out the significance of everything it has.

There’s a reason that "finding a needle in a haystack" is the cliche so often used when people talk about sifting through intelligence data. There is already a lot of data out there. And the more cautious the government gets about terrorism, the more likely it is to use that data to surveil as many people as possible in the name of "due diligence."

At a certain point, this makes it harder to find terrorists — not easier. And that’s especially true as the war on terror finishes its second decade, and actual terrorist organizations have begun to learn how not to get caught.

Under the "terror gap" proposal, the government is supposed to be limited by "reasonable suspicion." But you need information about someone to have a reasonable suspicion.

Surveillance is often discriminatory. The question is who it discriminates against.

The master terrorism watch list is supposed to cover both foreign and domestic "known or suspected" terrorists. But who gets labeled a terrorist in the US is a controversial question — namely, someone committing violence for his or her beliefs tends to be more easily labeled a terrorist if those beliefs involve Islam.

Of course, the fears of many gun-rights advocates and Republicans are very different. They’re much more concerned about a Democratic administration summarily deciding that all registered Republicans are suspected right-wing terrorists — and, therefore, summarily rejecting any attempt by any of them to purchase a gun.

As of March 2013 — when the Intercept acquired a copy of the (theoretically unclassified) criteria the government uses for watch listing — anyone who the government had "reasonable suspicion" to believe was a "known or suspected terrorist" was supposed to be in the TSDB. (For non-US citizens, the standard is lower for immigration purposes.)

But the guidebook doesn’t exactly set hard-and-fast standards for when there's enough evidence against someone to add him to the list.

This is by design. The government is trying to predict whether people who have never committed any acts of terrorism before will commit one now, and that involves a lot of what the government calls "predictive judgments" and its critics call guesswork.

Subjective guesswork often leads to racial profiling in domestic policing and counterterrorism alike, and there's evidence that this is happening in the government watch lists as well. The US city with the most entries on the master terrorism watch list is New York City, but the second most common is Dearborn, Michigan — which isn't a large town but does have one of America's largest Muslim populations.

The tension between using national-security systems for surveillance and using them for gun control is real. It’s perfectly logical: Systems change depending on what they’re being used for.

But the current state of national-security politics is that both parties are arguing over whether the system is being used to target the right people — who the real threats to America are — when, in fact, it might be the case that the system itself is a problem.

How fast 4 different guns can shoot and reload