Only Senate Minority Leader Harry M. Reid (D-Nev.), a onetime supporter of the NRA, has completely flipped his views on as many gun issues as has Sen. Robert P. Casey Jr. (D-Pa.), pictured. (Andrew Harnik/AP)

Sen. Robert P. Casey Jr. is one of the most unlikely evangelists for sweeping gun-control laws.

A decade ago the Pennsylvania Democrat won his Senate seat as a “pro life, pro gun” candidate, largely in the mold of his late father, a popular governor who fought his party’s liberal social values. His early stances on guns skewed right, including a 2009 vote allowing guns on Amtrak trains, and when a TV reporter in Pittsburgh informed him of the early details of the December 2012 elementary school shooting in Newtown, Conn., Casey gave what he now calls “the standard” answer.

“I don’t think new gun laws are going to change us,” Casey recalled Wednesday of his initial reaction to the massacre of 20 children and six educators.

By the time that weekend ended 3½ years ago, as Christmas approached and his wife and daughters accosted him over his views, Casey had a conversion. Casey has since embraced every major proposal to counter gun violence, including a renewed ban on assault weapons and enhanced background checks before gun purchases. On Monday, Casey was back in Pittsburgh to unveil legislation in the wake of the massacre of 49 people inside a nightclub popular with Orlando’s gay community that would forbid those convicted of a hate crime from purchasing weapons.

And Thursday night Casey will be among several dozen lawmakers honored by “Sandy Hook Promise,” a group pushing to end gun violence that was founded by some parents of the Connecticut children killed.

Casey faults himself for never really thinking about the gun issue until Newtown, coasting along with Pennsylvania’s traditional pro-gun views in a state where the National Rifle Association has held sway for decades.

When he returned to the Capitol after Newtown, Casey found it unacceptable that the NRA opposed any new laws.

“You’re saying that there’s nothing, nothing that the United States of America can do about that, absolutely nothing we can do, other than enforce the laws,” he said in an interview Wednesday, his voice breaking into a stutter at times fighting his emotion. “That’s when I said to myself: I can’t, I can’t, I can’t maintain this position anymore. It makes no sense.”

Casey’s switch has been a lonely reversal. Mass shooting after mass shooting, only a few senators have changed their views on gun laws.

Sens. Patrick Toomey (R-Pa.) and Joe Manchin (D-W. Va.), both formerly with A ratings from the NRA, offered tougher background checks on gun purchases, the main legislative vehicle for the weapons debate in the spring of 2013. But both senators remained against some other provisions, continuing to oppose banning certain weapons or restricting the size of bullet clips.

After Orlando, some Republicans have made overtures toward supporting a ban on U.S. citizens on federal terror watch lists from buying guns.

Only Senate Minority Leader Harry M. Reid (D-Nev.), a onetime supporter of the NRA, completely flipped his views on as many gun issues as Casey has. Reid is retiring at the end of this year, leaving Casey alone as a senator who completely reversed his positions and will now face voters with an entirely new view on guns.

Gun rights advocates immediately accused Casey of political expediency, switching his position just weeks after he had secured a new six-year term in his 2012 reelection. Kim Stolfer, chairman of the Pennsylvania-based Firearms Owners Against Crime, told the Philadelphia Inquirer that he was “a political opportunist seeking to curry favor with a public that knows nothing about the current level of laws.”

His 2018 reelection, coming in a mid-term with lower turnout, could be Casey’s most difficult race ever. Rather than its neutral position during his 2012 campaign, the NRA’s opposition is a near certainty, and the senator is steeling himself for the fight.

“In elections these days, you have to be prepared for the worst,” Casey said.

The NRA and most Republicans say that new restrictions would only hamper the ability of law-abiding citizens from purchasing weapons to defend themselves, as criminals and terrorists will use underground measures to acquire guns.

Casey rejects these arguments, in a manner completely out of character for the mild-mannered senator whose personality bends more toward his mother than his sometimes fiery father. He wants a response to the serial problem of “active shooter” situations to be dealt with akin to the effort to secure flights and the homeland after Sept. 11, 2001.

“Are we gonna surrender to that problem and just say, ‘You know there’s nothing we can do about this, it’s unfortunate but there’s nothing we can do about this.’ That is just unacceptable. I mean, it’s un-American. I don’t really care if people are offended by that,” he said.

Sitting on his desk are two newspaper clips, one with biographies of all the dead Sandy Hook children, another from 2015 breaking down of every mass shooting. He fixates on one child, Caroline Previdi, in part because she shared the name of one of the senator’s daughters.

A 6-year-old who looked after a kindergartner every day on the school bus, Previdi died in Sandy Hook. On Wednesday his new fixation became an 18-year-old honors student from Philadelphia’s West Catholic high school, a constituent visiting family in Orlando over the weekend.

“She’s dead,” Casey said, fighting back tears.

He joined a Democratic filibuster Wednesday on the Senate floor demanding votes on gun legislation, highlighting Akyra Murray’s death in Orlando.

He’s quick to blame Democrats for allowing energy to wane after the stalemate on the Manchin-Toomey bill. “Stay on it, and sustain it, that’s one of the biggest challenges, sustaining the focus, sustaining the urgency,” he said.

Casey isn’t sure what his father would think about his evolving positions on social issues, but noted that in his last year as governor, Casey Sr. supported the 1994 crime bill that included the last substantive gun restrictions enacted into law.

Still, he’s the first to admit how unlikely he is to have become so consumed by an issue that used to pass him by, despite the mass shootings in other towns before Newtown.

“I never thought this issue would have such an impact on my thinking and what I do,” Casey said.