Tom Sullivan was sitting in a third-floor committee room in the Colorado capitol last Wednesday when his phone lit up with news alerts describing a mass shooting in a Florida high school. In the five years since his son was murdered, with eleven other people, in an Aurora movie theatre, Sullivan has returned to the capitol’s wood-panelled hearing rooms and marble hallways more than twenty times. He has grown used to lawmakers rarely looking him the eye. He has learned to state his name for the record and to keep his testimony, recounting the worst day of his life, to roughly two minutes.

In 2013, Sullivan was among a small group of parents who lost children in Aurora or Newtown or Columbine who helped Democrats push Colorado’s legislature to enact tougher gun-control measures despite intense opposition from the N.R.A. After receiving the Parkland news alerts last week, Sullivan testified against the latest effort to roll back those measures, a proposal in the state’s Republican-controlled Senate that would expand concealed-carry rights in Colorado. The measure passed the committee in a party-line vote but this week was defeated, along with two other measures to loosen restrictions on guns, in the state’s Democrat-controlled House. For now Colorado’s stricter measures, including universal background checks and a ban on magazines of more than fifteen rounds, remain law.

Sullivan, who ran, unsuccessfully, for state Senate in 2016, says that Colorado shows that strengthening gun-control laws is possible, but keeping them in place requires political vigilance. “It is disappointing we have to play defense all the time,” he told me.

In the broader national narrative of N.R.A. invincibility, Colorado is an exception. Leveraging a statewide network of activists built over the preceding decade and funded by Michael Bloomberg and other national gun-control advocates, Democrats passed the gun-control measures when they controlled Colorado’s governorship and the state legislature. But they did not anticipate the backlash that allowed Republicans to win control of the state Senate by ousting two Democratic state senators in recall votes and winning a third seat after a Democrat resigned.

The governor, John Hickenlooper, a brewpub owner turned politician, said in an interview this week that after the Aurora shooting he thought he could foster a bipartisan consensus regarding universal background checks. He recalled having cordial conversations about gun laws with Second Amendment experts and Republican donors heading into the 2013 legislative session. “Every single Republican I talked to supported universal background checks,” Hickenlooper said. “So, in my naïveté, I thought it would whiz right through.”

Further Reading New Yorker writers respond to the Parkland school shooting.

As the bills moved through the legislative process, hundreds of gun owners clogged the halls of the capitol. Representative Rhonda Fields, the Democratic lawmaker who sponsored the two main bills in the House, received death threats, as did two pro-gun-control lobbyists. Magpul, which manufactures high-capacity magazines, threatened to shutter its northern Colorado factory if the magazine limit became law. (The company followed through, relocating and moving roughly a hundred jobs to Cheyenne, Wyoming, in 2015.) Several of Colorado’s sheriffs, mostly from rural counties, later sued the state in an effort to overturn the new laws. The lawsuits failed, but the N.R.A. and local gun-rights groups persist in their yearly efforts to reverse the laws.

Will Coyne, one of two lobbyists hired by Bloomberg’s organization Mayors Against Illegal Guns, described 2013 as a year-long political epiphany. “I realized I did not fully understand politics in America, and how intrinsically the segment of society that cares about gun rights, how deeply they believe that, how much gun rights are wrapped up in their political and cultural identity,” he said.

John Morse, a former Colorado Springs-area police chief who passed the measures when he served as the president of the Colorado state Senate, paid the heaviest political price. Although Morse expresses no regrets, he said his recall after the gun-safety measures passed created an excuse for other Democrats not to act. “We beat the N.R.A. in Colorado, and then what happened was they beat us,” he said. “They rode me out of town on a rail, and now there’s not a Democrat in the country that has the stomach to do what needs to be done.”

Coyne praised Morse, saying “he pushed as far as he possibly could, and he did it because he thought it was the right thing to do, and he did it without fear of consequence. In my career, I’ve never seen another politician act like that.”

Democrats also initially underestimated the intensity of support for some gun-control initiatives on their own side. The magazine ban gathered steam when Hickenlooper, known for his political caution, unexpectedly got behind it. “It was tangible in a way background checks weren’t,” Coyne, the lobbyist, told me. “People could hold the magazines and see the difference in size. It actually galvanized our side in a way we hadn’t expected, and it gave us more momentum.”

Gun-rights advocates, meanwhile, seized on the legislative battle to boost their own membership rolls and organize protests (including a day when gun owners circled the capitol in cars while honking their horns). Hickenlooper contends that as mass shootings continue to occur, broader public frustration will grow. With significant federal reforms unlikely, he predicted that more states will eventually enact reforms similar to Colorado’s. “I think those folks, the people that really thought that this was too great a cost to pay—now history says, if anything, we should have pushed a little further,” Hickenlooper said. “While I’m sure there are things in the process we could have changed or improved, I don’t have any anxiety or self-doubt that what we did was important and right.”

A Quinnipiac Univerity poll released this week found that sixty-six per cent of American voters supported stricter gun control laws, a fourteen per cent increase since 2015. Coyne said that his private polling from last December showed support for Colorado’s gun laws trending up, with thirty-one per cent supporting the laws in 2014 and thirty-eight per cent supporting them in 2017.

Tom Sullivan returned to the capitol this week to testify against a measure designed to roll back the state’s high-capacity magazine ban. He cautioned the young activists of Parkland that “anger is only going to take you so far in this,” but lauded their resolve. “I applaud those Florida students for what they’re doing,” he said. “I welcome them into the fight, because that’s what this is going to be.”