© Julia Robinson for The Wall Street Journal

SEGUIN, Texas—Stephen Willeford was widely hailed by supporters of the Second Amendment as the model “good guy with a gun” in 2017, when he grabbed an AR-15 rifle and pursued and shot at a gunman killing churchgoers in Sutherland Springs, Texas.

Two and a half years later, the gregarious plumber has embraced the moniker Good Gun Guy Wille in speeches, church security training and now in a campaign for local county commissioner. If he wins, he wants to make Wilson County a “Second Amendment sanctuary” that would defy any restrictions state politicians might put on guns.

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“I never had such a voice,” Mr. Willeford said. “Now people are willing to listen to me.”

He is one of three men in Texas touched by shootings at churches who are drawing on their experiences to run for public office in March’s Republican primaries.

Pastor Frank Pomeroy, who lost his teenage daughter and 25 other congregants in the Sutherland Springs church shooting, is challenging a popular Democratic state senator.

In North Texas, firearms instructor Jack Wilson was already a candidate for county commissioner when he took down a gunman Dec. 29 at the West Freeway Church of Christ.

They aren’t the first people to run for office after being involved in gun violence. But the Texas candidates stand out because they are using their experiences to boost gun-rights campaigns.

Mark Kelly, the husband of former Democratic U.S. Rep. Gabrielle Giffords and a former astronaut, is running in Arizona for the U.S. Senate as a Democrat, calling for greater gun control. Ms. Giffords was shot and six others were killed in a shooting spree in 2011. In Georgia, a Democratic gun-control advocate, Lucy McBath, whose son was killed in a shooting after an argument at a gas station, won a U.S. House seat in 2018.

J.T. Lewis, whose younger brother was among 26 killed at Sandy Hook Elementary School in 2012, is running for Connecticut state senate as a Republican. Mr. Lewis’s platform calls for greater school safety measures, including armed guards, and he supports expanding background checks for those who want to buy guns.

Harel Shapira, a University of Texas sociology professor who does research on guns and right-wing politics, said he isn’t surprised by the campaigns in his state.

Gun ownership has become increasingly tied to political identity over the past two decades while, simultaneously, gun issues have come up less on a federal level and more in state and local politics, he said.

“The response to [mass shootings] is so polarized, and each becomes a moment for people on both sides to stake their claims,” Prof. Shapira said.

Four of the nation’s 10-deadliest mass shootings ever were in Texas, and gun-control advocates, most of them Democrats, argue that Texans are fed up. A recent Dallas Morning News/University of Texas poll found that 86% of registered voters in the state support background checks for gun buyers and more than half support a nationwide ban on high-capacity ammunition clips with more than 10 bullets.

Near the site of the shooting in Sutherland Springs, Mr. Pomeroy recently campaigned and registered voters in the small city of Seguin, at an arts fair next to booths selling concealed-carry handbags. Other parts of the district, which extends along the U.S.-Mexico border, are more liberal, and Mr. Pomeroy, the only Republican running in the primary, is considered by political consultants to have long odds. His opponent in the general election, Judith Zaffirini, is a 33-year Democrat incumbent who was the Lone Star State’s first Latina state senator.

Mr. Pomeroy said that though the district is overwhelmingly Democratic, its residents are largely churchgoers and gun supporters. His top issues are opposing abortion and supporting gun rights, though he said he is open to talking about some firearms restrictions.

“As Texans, we believe in personal responsibility, and we want to make more critically thought decisions, even though, obviously, when we see these shootings happen and see that parents have lost children, it tweaks our emotions,” he said.

Ms. Zaffirini declined to comment on her opponent. “Under no circumstances should we politicize this issue,” she said of gun violence.

In Hood County, a red bull’s-eye sits above Mr. Wilson’s name on his campaign signs that read, “Make sure your vote is on target.” The signs were printed before Mr. Wilson shot and killed a gunman who killed two people at his church in December. He said he frequently has to explain the events to people who say he is exploiting the calamity.

In a heavily Republican district, the winner of the four-person primary is likely to take the seat on the commission. Until December, local political experts expected a state representative who is leaving the legislature to be the likely winner of the race. But Mr. Wilson, who won the Medal of Courage, the highest honor given to civilians in Texas by the governor, now also has name recognition. In public spaces, he is often greeted with handshakes and expressions of thanks from strangers.

Mr. Willeford said he is proud that the volunteer security team at Mr. Wilson’s church was formed shortly after his pursuit of the gunman in the Sutherland Springs massacre, who died from a self-inflicted gunshot wound. He wants the three men’s political involvement to spur more people to arm themselves to defend their communities.

“I hope it’s a trend,” he said. “Let’s be proactive.”