Rep. Brian Mast’s conversion to gun-control advocate began next to a pool as he tried to find solace during last week’s congressional recess. The Florida Republican’s children were playing with dozens of other kids, but Mast could not stop thinking about the 17 students and faculty killed at a high school in his former home, Parkland. One of them, the assistant football coach, was a friend.

Mast, 37, was an Army bomb technician whose job was to run toward trouble as comrades ran away. He lost both legs “trying to keep my brothers safe.” On this particular day, his 9-millimeter handgun was tucked nearby, as it nearly always is when he travels back home.

Mast looked up at a hotel a few hundred yards from the pool where he sat. He thought about the Las Vegas massacre, when a gunman opened fire from the 32nd floor of a casino hotel, leaving 58 people dead and hundreds more injured.

With gun control advocates facing off against gun rights groups after Parkland, each side is trying to set goalposts for what is - and isn't - on the table. (Jenny Starrs/The Washington Post)

“I’m a sitting duck,” he recalled thinking in an interview Monday evening, moments after the House held a moment of silence for the Parkland shooting. Mast is a Bronze Star recipient — the ultimate “good guy with a gun,” as gun rights supporters like to say. But his handgun and concealed-weapon permit would have been no match for someone firing a military-style weapon from high above.

“He could take me out, and my concealed-carry does me no good,” he recalled thinking that day.

That’s when Mast started thinking, for the first time in his brief political career, about how federal gun laws should change. A few days later, he published a column in the New York Times declaring his support for a ban on “assault or tactical” firearms; a requirement for background checks on all gun purchases, including online and private sales; and a ban on accessories such as “bump stocks,” which allow guns to simulate the rapid fire of fully automatic weapons.

Mast is heading into his first reelection this year, a potentially difficult fight in Florida’s 18th Congressional District, which includes part of Palm Beach County and sits about an hour north of Marjory Stoneman Douglas High School, where the shooting unfolded on Valentine’s Day.

It doesn’t take rocket science to speculate that Mast could benefit in the general election from independent-minded voters who appreciate his deviation from standard Republican orthodoxy on gun laws. But he just as easily might infuriate conservative activists who, in 2016, helped swing a district that leaned slightly Republican to give President Trump a victory margin of more than nine percentage points.

The easy move for Mast might have been to keep his head low and wait for the intensity of the issue to fade away, then focus on touting the Republican tax-cut plan that helps his wealthy constituents.

Instead, he reacted to the issue like he did when soldiers found improvised explosive devices on the battlefield. “I’m not trying to say this in like a heroic way or a macho way or something like that; I never worried about being a casualty on the battlefield. Literally, I never did,” said Mast, who walks with the use of two prosthetic legs and a cane.

“When they were going the other way, I was going towards it,” he said of his 12 years in the Army.

Mast got his first taste of reaction Sunday evening at a civic event at, of all places, Mar-a-Lago, the president’s resort just a few miles outside Mast’s district. Trump was not there, but plenty of local Republicans were — and they let him know what they thought.

Some whispered to him: “Spot on, you’re exactly right,” he recalled.

Others politely disagreed: “Listen, I don’t agree with you, but I’m so glad that you had the courage to say it, to step out there and say what you believe when you know people are going to beat you up over it.”

A few close friends and political supporters turned away without a greeting, which Mast interpreted as a political shunning for his apostasy.

Stoneman Douglas is about 40 miles from Mast’s home, but it holds a special place in his heart that helped prompt the deep reflection leading him to his current stance on gun laws.

In 2010 in Afghanistan, Mast stepped on an IED while trying to clear a path for Army Rangers, beginning a long recovery process, including a stint at Walter Reed National Military Medical Center. In 2013, the Michigan native moved to Florida — Parkland, to be precise.

Word got around that a wounded warrior was moving in, so Aaron Feis gathered up some of the school’s football and lacrosse players, unprompted. The assistant football coach, who was also a security guard, led the players in helping their new neighbor’s family unpack their boxes.

Two weeks ago, as the bullets started flying, Feis was ushering students to safety when he was felled by the gunman’s bullets. Mast attended Feis’s funeral, where the coach was honored for “making the biggest difference in every person’s life, certainly that day,” Mast said.

“I think it was one of those things that helps a man like me to have courage,” he said.

Mast, who has three children, began writing out his thoughts on gun control after sitting by that pool — and after never having really thought about the issue despite receiving nearly $5,000 in contributions from the National Rifle Association.

It took three days to compose. “Writing it and writing it, collecting my thoughts,” he said. “And writing it and collecting my thoughts.”

Finally, Mast gave House GOP leaders notice of what was coming and took his stand, becoming a rare Republican convert to some of the country’s more controversial gun-control proposals. To be clear, Mast continues to support the rights of gun owners to carry a concealed weapon, as he regularly does.

Based on the reactions at Mar-a-Lago, Mast is bracing for an opponent in the Republican primary, which isn’t until August in Florida — a lot of time to gin up conservative opposition to a first-term incumbent without roots in the district.

“I said what I thought was right. I said what I believe,” Mast said, his cane resting on the marble stairs just off the House chamber. He wants his friends and enemies alike to know this about him: “I don’t take the easy road.”

Mast has already faced tougher paths anyway.

“If you think I’m going to worry for one second about being a political casualty, when I think I can save a life, it’s not going to happen.”