In the wake of the Orlando attack, Republicans have expressed a willingness to compromise on the purchase of guns by people on terrorist watch lists but legislation is still far from a certainty. Here’s why

Democrats ended a 14-hour filibuster in the Senate overnight with the promise of some progress on proposed gun control legislation. Republicans leaders agreed to take a vote on amendments to expand background checks and ban gun sales to those who are on the government’s terror watch list, closing what is known as the “terror gap”.

It’s unknown how Republicans would vote on the proposals. But in the wake of the nightclub shootings in Orlando, a small handful of them in the Senate are now showing greater willingness to compromise on the terror proposal than they did last December, when rival measures from the two parties to close the terror gap in the wake of the San Bernardino shooting both failed.

Meanwhile, Donald Trump, the presumptive Republican nominee for the White House, is scheduled to talk to the NRA on Thursday to help formulate his own policy position on the so-called terror gap – a not-so-subtle acknowledgement of the extraordinary power that the gun lobby wields over legislators and candidates for high office alike. Here’s what you need to know about the terror gap.

What is the terror gap?

The terror gap is the notion of a legislative hole whereby US citizens can purchase deadly firearms even if they are under investigation for suspected terrorist activity. The Government Accountability Office found that between 2004 and 2014, some 91% of suspected terrorists who attempted to buy a gun – 2,043 out of 2,233 – succeeded.

Attempts to close the gap, going back to the George W Bush administration, have repeatedly failed despite broad public support because of pressure on congressional legislators from the NRA.

Is this another case of partisan gridlock?

Not entirely. The Bush and Obama administrations have both supported the same set of proposals, and the legislation most hotly contested by the NRA has been proposed jointly by Dianne Feinstein of California, a Democratic senator, and Peter King of New York, a Republican congressman. That said, Republicans tend to be in lockstep with the NRA more consistently than Democrats. Fifty-four senators voted against the Feinstein-King legislation last December, 53 of whom were Republicans. Just one Republican, Mark Kirk of Illinois, voted in favor.

The FBI has different ways of categorizing people it is investigating for possible terrorist ties. In addition, there is a federal no-fly list of people deemed too dangerous to be allowed to board an airplane. Some in the Senate, including Republican Pat Toomey, want to create a single consolidated list and have it subject to the authority of the foreign intelligence surveillance, or Fisa, court.

What are the ideas for fixing it?

Feinstein and King want to give the Department of Justice, which includes the FBI, sole discretion over which terrorist suspects get to purchase weapons and which do not – in part so that the FBI has the option of allowing sales to go ahead as part of their investigative process. Anyone who feels unfairly targeted would still have a chance to appeal against any denial of gun rights in the courts.

The NRA’s preferred approach, currently championed in the Senate by Republican John Cornyn, would require the government to respond to a contested gun sale by filing a brief in federal court, offering the targeted individual the opportunity to make his or her case in response, and convincing the judge to rule within 72 hours. Without fulfilling all of these conditions, the sale would go ahead. The president of Michael Bloomberg’s group Everytown for Gun Safety said this week: “The Cornyn bill has an absurdly high standard that applies only to people who are proven to be about to commit a terrorist act. At that point, we shouldn’t be debating about terrorists’ gun rights – just about the quickest way to incapacitate them.”

Among those seeking a compromise is Toomey, a Republican facing re-election in Pennsylvania, a key swing state. He wants to address concerns about government overreach leading to people being put on a list erroneously by consolidating the many different ways the FBI has of tracking terror suspects, as well as the federal no-fly list, and putting it under the authority of the Fisa court.

If closing the terror gap is so popular, what is the argument against it?

Detractors on the left and the right are worried that a list of terror suspects used to curtail individual liberties would be prone to abuse and inadequate constitutional safeguards. The no-fly list has come under fire for exactly this criticism. Many people have not been informed that they are on the list, making it difficult or impossible to get themselves removed, and many others have not been given a reason for their inclusion. The American Civil Liberties Union is engaged in a five-year-old legal battle to challenge the constitutionality of the no-fly list and would accept it as a basis for denying individual gun rights only “with major reforms”.

The NRA, for its part, cites the risk of governmental overreach as a cornerstone of its argument for subjecting any contested gun purchase to the scrutiny of a judge. In principle, the NRA says it wants to keep guns out of the hands of criminals and terrorists, but in practice it has set almost impossibly high bars to making this happen.

If the terror gap had been closed, would it have prevented the massacre in Orlando?

Probably not. The Feinstein-King legislation as written last December would not have covered Omar Mateen because the FBI twice put him on a list of suspected terrorists and then took him off again. They now have new language, tailored to the events in Orlando, to extend a weapons purchase ban for five years after an individual is put on a terrorism suspect list.

Of course, there are many other ways to obtain an assault rifle and a machine pistol such as the weapons used at the Pulse nightclub than by going into a gun store – for example, at a gun show, where there are no background checks. Another bill that will now go up for a vote to require background checks at gun shows is much less likely to pass.

Are the chances of closing the gap now any greater than they were after any of the other recent atrocities?

Slightly, but don’t hold your breath. Senators such as Toomey and Rob Portman of Ohio, also facing re-election this year, are indicating an interest in finding compromise language. Feinstein, however, said on Wednesday that her own efforts to find a middle ground were not working. “I don’t think it’s going to work out,” she said.

Why is Trump talking to the NRA before announcing his own position on the issue?

Because, as Ari Freilich, a staff attorney with the Law Center to Prevent Gun Violence, put it: “The NRA sets the term of the debate.” That applies even to a presidential candidate who famously claims to be beholden to nobody. The NRA has endorsed Trump, and Trump is doing what many, many other NRA endorsees have done before him.