The shooting at Robb Elementary School in Uvalde, Texas, on Tuesday was the second-deadliest school shooting since 1970. But in the aftermath of Columbine , Parkland, Sandy Hook and the nearly 1,000 other school shootings since 2012, advocates for nationwide gun control measures, like universal background checks and banning assault rifles, have run into political realities that have stopped these efforts from moving forward.

In the aftermath of the shooting in Uvalde, some elected officials have indicated there could be legislative change. Senate Minority Leader Mitch McConnell said he wants Republican senators like Texas' John Cornyn to work with Democrats to find common ground on a legislative response to the shooting. But years of disagreements on gun violence solutions in the aftermath of deadly shootings across the country for decades have led to no major reforms on the matter.

"I think the likelihood the Senate will act is still vanishingly small, but it's no zero," Bob Spitzer, author of "The Politics of Gun Control" told NPR. "It's not clear to me why Mitch McConnell would be interested in allowing that to happen... but it's not impossible just because of the atmospherics of right now."

Historically, Republican lawmakers have generally said they wouldn't back measures that limit firearm rights, and in the last few days several GOP lawmakers have reiterated that sentiment.

"Guns are not the problem. ... We've had guns forever. And we're going to continue to have guns," Republican Sen. Tommy Tuberville said Wednesday .

Still, lawmakers like Democratic Sen. Chris Murphy remain hopeful.

"Maybe I'm a fool for being the eternal optimist, but I'm just gonna stay at it for these next few days, the next week," Murphy told to NPR's All Things Considered.

Passing major gun control measures on a federal level is possible — it's been done before . And gun control advocates point to more pieces of progress that have have been passed on state levels in the last decade. But over the last few decades, gun rights has become an even more polarized topic among elected officials in Washington.

Here's a look at some of the political barriers that have prevented the debate from progressing to action.

There are limits on what the president can do

If a divided Congress can't take action on gun violence, even when there is public opinion behind it, there are often calls for the executive branch to act within its own authority. Past presidents have taken action on their own to help curtail gun violence, but major reforms require legislative action from Congress. For example, former President Trump banned bump stocks after the Las Vegas shooting in 2017 while legislation that would do the same never gained traction in Congress.

So far in his administration, President Biden has signed a series of executive orders that tackle ghost guns and braces on AR-15 pistols. And former President Obama also took executive actions that were meant to more strictly regulate gun purchases. But experts point out that those actions did little in reality to curtail mass shootings.

Furthermore, executive actions can be undone by the next administration. When Trump came into office in 2017, he revoked Obama's order that tried to make it harder for those with mental illnesses to purchase weapons.

The Supreme Court's interpretation of the Second Amendment

In recent decades, the U.S. Supreme Court has ruled on two gun rights cases that have effectively expanded its interpretation of the Second Amendment, which enshrines the right to bear arms.

In the 2008 case District of Columbia v. Heller, the court determined, for the first time, that an individual had the right to possess a handgun in their home. In the 2010 case McDonald v. City of Chicago, the court built on Heller and said that state and local gun control measures must respect that right.

Spitzer says that despite the court's ruling in Heller, it didn't set a precedent of loosening most gun restrictions that already exist. But an upcoming opinion from the Supreme Court involving a New York state law does have the potential to broaden gun rights, Spitzer said.

That case involves a challenge to the state's "proper cause law" requiring that people applying for a license to carry a concealed weapon outside the home show that they have a concrete need for self-defense — or they're going hunting or to target practice. In hearing the case in November, the conservative majority seemed ready strike down New York's law. Several other states have similar restrictions for carrying guns in public.

"I think they're going to redefine gun rights by expanding them to outside the home in some manner," Spitzer said.

Pro-gun lobby has spent millions

Pro-gun rights organizations like the NRA and Gun Owners of America, which have spent tens of millions of dollars on lobbying and contributions to political candidates they consider to be on their side of the debate, most of them Republicans.

In the 2020 presidential race, the NRA spent more than $16 million to back Trump. It spent more than $12 million on congressional races, far surpassing direct spending by gun control groups.

For decades, the NRA has been a major block to not only passing gun control legislation, but to even researching the negative effects of owning guns.

A culture of hopelessness

On top of policy barriers, the incessant number of mass shootings in the United States have enabled a culture that has begun to see gun violence as a norm. It's helped create a culture of hopelessness, Peter Ambler, political director at the gun control advocacy group Giffords, said.

"One of the broader barriers is hopelessness, which is the NRA's chief political product... they use hopelessness to stymie progress," he said. Hopelessness sort of paralyzes the response."

Some advocates want to frame gun control as a public health crisis

While political debate over how to respond to the Uvalde shooting has already begun, some gun control advocacy groups are already sending the message to Congress that the endemic of gun violence in the country needs to be treated, and legislated, as a public health crisis that addresses root causes of violence.

"Our policy solutions should start from a public health perspective. And that means we can't just focus on the firearms and the hardware," Greg Jackson, the executive director of the Community Justice Action Fund, and a gun violence survivor himself, said to NPR.

"While those efforts will be helpful, the reality is that there are still 400 million guns in our country already. There's more guns than people. Most neighborhoods like mine are already saturated with firearms and so we have to make sure there's a comprehensive strategy to reduce violence," Jackson said.

He says his organization is already communicating the message to Congressional leaders, including Senate Majority Leader Chuck Schumer, that bills focused on regulatory gun control and crime control aren't the only solution to prevent violence. He encourages lawmakers, and the public, to see combating gun violence as something that can be tackled on multiple fronts, including by addressing trauma and other roots of violence.

"If we're just focusing on the Second Amendment debate, then we're over-simplifying this issue," he said. "There are dozens, if not more, of legislative approaches that could be considered in this moment."

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