A Colt .45 semi-automatic pistol in Manassas, Va. (Karen Bleier/Agence France-Presse via Getty Images)

This week, Sen. Chris Murphy (D-Conn.) staged a 15-hour filibuster on the Senate floor to secure a vote on legislation that would make it harder for people on the terrorist watch list to legally purchase guns in the United States. The effort underscores a strange fact about U.S. firearm laws: Being placed on a federal terrorist watch list is no barrier to passing a gun background check.

"Membership in a terrorist organization does not prohibit a person from possessing firearms or explosives under current federal law," as the Government Accountability Office concluded in 2010. And indeed, plenty of people on these watch lists do purchase firearms: Between 2004 and 2015, people on the terrorist watch list passed federal gun background checks no fewer than 2,265 times. At least three of those background checks involved the purchase of explosives. Only 212 attempted purchases were blocked, a successful purchase rate of over 90 percent.

Last year alone, people on the terrorist watch list attempted to purchase guns 244 times. Of those, 223 attempts were successful.

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The issue of allowing people with suspected ties to terrorism to buy guns has come to the forefront of the political debate after the FBI revealed that the Orlando nightclub shooter was placed on the federal terror watch list but was ultimately removed. That shooter was able to go to a gun store and pass a background check allowing him to purchase an assault rifle just days before the attack.

Civil libertarians have long voiced concerns over the terror watch lists. "The government’s watch listing system amounts to an unchecked exercise of power over innocent citizens and non-citizens alike," the American Civil Liberties Union wrote in a March 2014 brief. "Countless individuals have been placed on watchlists based on intelligence of unknown reliability and according to standards that are either secret or so broad as to be formless."

The FBI's terror watch list had about 800,000 people on it as of 2014. Roughly 40,000 of them, or 5 percent, were U.S. citizens.

Gun-rights groups, such as the National Rifle Association, are opposed to the idea of denying citizens a fundamental right — to bear arms — on the basis of their placement on an extra-judicial watch list that they say is exceedingly broad and riddled with errors. The NRA says that if a terror suspect attempts to purchase a gun, authorities should be notified and the purchase put on hold to allow authorities to investigate.

A bill sponsored by Sen. John Cornyn (R-Tex.) would do just that, giving authorities 72 hours to find probable cause to block the sale of a gun to an individual on a watch list. If no such probable cause were found, the sale would proceed.

But Democrats say that Cornyn's bill would have little real-world effect, given the difficulties of launching a probable cause investigation, completing it and presenting it to a judge in 72 hours. A competing Democratic measure, introduced by Sen. Dianne Feinstein (D-Calif.), would give the U.S. attorney general discretion to block gun sales to anyone who has been investigated for a terror offense in the past five years. According to Feinstein's office, the FBI's watch list would be taken out of the process entirely.

When similar measures were put before Congress in December following the terrorist shootings in San Bernardino, Calif., both measures failed — Feinstein's bill on account of insufficient Republican support, Cornyn's because of a lack of support from Democrats. The measures currently before the Senate appear to be headed for a similar fate.

It's a classic case of partisan paralysis in the face of overwhelming public demand for change: A recent Huffington Post-YouGov poll found that 86 percent of Americans support a law preventing individuals on the watch list from buying guns. Democrats and Republicans are united on the question, with 87 percent of each group supporting such a ban.