With each mass shooting, the elapsed time between shared horror and polarized debate about how to prevent future atrocities narrows a little further.

One argument, articulated by likely Democratic presidential nominee Hillary Clinton after an ISIS-inspired gunman killed 49 people in an Orlando, Florida, nightclub, holds that the U.S. needs to strengthen its gun laws.

Another premise, put forward most prominently by the National Rifle Association, argues that the only constitutionally valid way to stop terrorist shootings is to arm as many citizens as possible.

Since the attacks of September 11, 2001, the NRA has forcefully advanced this position with slick ad campaigns, and through a continued push for legislation that normalizes the presence of firearms in public life.

Its tactics have proven remarkably effective. In the past 15 years, no new major federal gun restrictions have been enacted, and the NRA has helped propel dozens of new state-level laws that make it lawful for people to carry guns in places that were once off limits, including college classrooms. It has also fought legislation that would close the “terror gap” by stopping gun sales to people on the FBI’s terrorist watch list.

“The NRA tells people to not let the government use 9/11 as a subterfuge to take away their guns,” Representative Peter King, a New York Republican, tells The Trace. “It tells them people need guns to protect themselves against terrorists.”

On Wednesday, the NRA appeared to offer support for an effort to close the terror gap that would give the Justice Department 72 hours to collect and present evidence that a purchaser on the terrorist list has committed an act of terrorism, or will in the future. Democrats have assailed the proposal as unworkable.

In December, one day after attacks by ISIS sympathizers in San Bernardino, California, left 14 dead, a competing Democrat-led terror gap bill, which would not require the Justice Department to launch an investigation to deny a purchase, died in the U.S. Senate on a mostly party-line vote.

The attacks of September 11 greatly elevated anxieties about domestic terrorism. The FBI reported that it had conducted almost half a million more background checks for gun purchases in the first six months after the attacks than it had in the same period the previous year.

The NRA spotted an opportunity, and made an aggressive push to expand gun rights, arguing that a well-armed populace is the best way to prevent attacks of any kind. In states like Virginia and Georgia, it campaigned for bills that sought to chip away at gun-free zones, allowing people to carry firearms in bars, churches, and government buildings.

“It’s a natural feeling that after 9/11, people want to be proactive and take necessary actions to protect themselves and their loved ones in these uncertain times,” Andrew Arulanandam, an NRA spokesman, told ABC News in 2002.

At that year’s annual meeting, NRA Executive Vice President Wayne LaPierre warned members of a secret plot to clamp down on Second Amendment rights, chastising liberals who he said had suggested that terrorists would eventually carry out their attacks with guns.

For the “gun ban lobby” and lawmakers like Democratic Senators Charles Schumer and Dianne Feinstein, the attacks were “a godsend,” he said, a way to market “terrorism as the new reason to ban your guns.”

He continued: “These groups began painting a fictional picture of wild-eyed terrorists equipping rogue armies through small town gun shows in the heartland of America.”

Jeb Bush, then the Florida governor, took up the call at the NRA’s 2003 meeting. “The Second Amendment is the original Homeland Security Act,” he declared.

During Bush’s tenure as governor, Florida passed a number of laws that expanded gun rights. The most notorious of these, “stand your ground,” explicitly addressed self-defense. Crafted by the NRA, the law extends to the public sphere the right to use a firearm against a perceived threat. The final legislation was eventually copied by 21 other states that passed their own version of the law.

In 2007, the NRA perceived a new threat to gun rights when a group of bipartisan lawmakers in Congress, led by King, introduced legislation to prohibit people on the FBI’s terrorist watch list from buying firearms.

The Government Accountability Office had found that suspects on the FBI’s terrorist watchlist were legally purchasing firearms because they were not being reported to the federal background check system. The NRA mounted a vigorous campaign against the “terror gap” legislation, arguing that the bill “would allow arbitrary denial of Second Amendment rights based on mere ‘suspicions’ of a terrorist threat.”

That bill has been defeated in every session since 2007.

“The NRA has been successful in fomenting fear of retribution in members of Congress, particularly from rural states,” says David Axelrod, a former senior strategist for President Obama. “They fear the money. They fear the mobilization of voters for whom opposition to any gun controls is the primary voting issue. They fear losing their seats.”

As of 2015, more than 2,000 suspects on the FBI’s list had legally purchased firearms, according to the GAO. This doesn’t mean that these purchasers are necessarily a threat: The Department of Justice has found that thousands of individuals have been placed on the watch list erroneously.

Since the tragedy in Orlando, there has been a renewed push to address the terror gap. There are now two competing measures: the version introduced by King and his upper chamber partner, Senator Feinstein, and an alternative proposal offered by Senator John Cornyn, a Texas Republican.

It is Cornyn’s proposal, which would require the Justice Department to demonstrate probable cause to block a sale within 72 hours, that the NRA has appeared to endorse.

The NRA positions itself as a leader in the fight against jihadists. In September, the group launched a video campaign called “Freedom’s Safest Place.” A prominent spokesman is Marcus Luttrell, the former Navy Seal whose autobiography served as the basis for the film “Lone Survivor.” In one of the videos, he looks square at the camera and addresses an imaginary terrorist. “I will say what I think, worship according to my beliefs, and raise my children how I see fit,” he says. “And I defend it all with the Second Amendment to the Constitution of the United States.”

Republican candidates for elected office have inserted the NRA’s rhetoric into their campaigns. In May, presumptive Republican nominee Donald Trump devoted a portion of his speech at the NRA’s annual convention in Louisville, Kentucky, to railing against gun-free zones, parroting the gun organization’s line that they invite mass shootings and terrorist attacks.

The day after the massacre in Orlando, Trump returned to the same theme.

“By the way,” he said in an interview, “if you had some guns in that club the night that this took place, if you had guns on the other side, you wouldn’t have had the tragedy that you had.”

In Orlando, the weapons used by the shooter — a military-style rifle similar to an AR-15 and a 9mm semi-automatic handgun — were purchased legally just days before the attack.

On Monday evening, the NRA’s chief lobbyist Chris Cox wrote a column in USA Today in response to the events in Orlando. Cox argued against calls for gun restrictions in the wake of the deadliest mass shooting in U.S. history. “Radical Islamic terrorists,” he wrote, “are not deterred by gun control laws.”

[Photo: AP/Phelan M. Ebenhack]