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Taunts and threats escalated as rumors about Jacob Reitan’s sexual orientation swirled around his southern Minnesota high school during his senior year in 1999.

Someone chalked a slur on his driveway in giant letters. His mother recalled being horrified by anonymous mail that arrived at their home, including one message that said her gay son would be better off dead.

After the teenager found his car window smashed in the school parking lot, he told officials at Mankato West High School that he intended to come out of the closet and sought their support to start a gay-straight alliance club.

The principal made an unconventional decision in the choice of a faculty adviser for the club, which the high school had never had before in this relatively conservative city: Tim Walz, a geography teacher who was also a football coach. Mr. Walz readily agreed, and the choice came as a relief to Mr. Reitan.

“It was important to have a person who was so well-liked on campus, a football coach who had served in the military,” said Mr. Reitan, now 42. “Having Tim Walz as the adviser of the gay-straight alliance made me feel safe coming to school.”

That early chapter from Mr. Walz’s teaching years helped shape his stances as a politician and reflects how much Mr. Walz’s partnership with his wife, Gwen, has shaped his work over many years. Heavily influenced by Ms. Walz, who was Mr. Reitan’s teacher and someone the student confided in, Mr. Walz went on to make gay rights a signature issue in his political career, supporting same-sex marriage while representing his mostly conservative, rural congressional district long before mainstream Democrats were.

As a member of Congress, Mr. Walz, who served in the National Guard for 24 years, was a leading proponent of repealing the ban on openly gay people serving in the military.

As governor of Minnesota, he signed a bill prohibiting bans on books in schools and public libraries, which came as conservatives in states around the country have challenged books, including some with gay characters. He signed a bill banning conversion therapy for gay and transgender people. And as many conservative states banned transition-related medical care for transgender youths, Mr. Walz signed a bill that protected transition treatment in Minnesota, and shielded those traveling to the state for such treatment from any legal consequences.

“They want to put bullies in charge of your health care,” Mr. Walz said during his State of the State address in 2023, celebrating the transgender medical care measure. “We want to put you in charge of your health care and put bullies in their place.”

His outspoken support for gay and transgender rights has galvanized liberal factions of the Democratic Party in recent days, and has become a focus of criticism by conservatives, who consider Mr. Walz a far-left candidate partly because of his positions on L.G.B.T.Q. issues. Former President Donald J. Trump recently described Mr. Walz as a “radical left person,” adding that he was “heavy into the transgender world.”

Mr. Walz’s stances can be traced, in part, to the early chapter in Mr. Reitan’s life, in the late 1990s, when he was a high school student in Mankato.

Mr. Reitan, now a lawyer and gay rights activist, recalled being bullied over his sexual orientation as early as the seventh grade, when he opened a book from a classroom shelf and found a note scribbled inside: “Jake Reitan is gay.”

“It caused me a significant amount of anxiety,” he said.

In the years that followed, the few mentions of gay people that he heard about, whether in the news or from people around town, were contemptuous, he recalled. In 1997, Minnesota lawmakers passed a bill limiting marriage to a man and a woman. The comedian and talk show host Ellen DeGeneres had not yet come out of the closet, and the sitcom “Will & Grace,” which featured openly gay characters, had not yet become a cultural touchstone.

Mr. Reitan said he was astonished by what happened one day in his 10th grade English class. Out of the blue, his teacher, Ms. Walz, announced that her classroom was “a safe space for gay and lesbian students,” he recalled.

Mr. Reitan, then 15, said he froze, his heart pounding, wondering if the remark had been meant for him.

“I did not know any gay people, and there were no gay people on TV then,” he said. “It was very much a closet of one.”

Long before there was a national debate over what — if anything — teachers should be allowed to say about sexual orientation and gender identity in the classroom, Ms. Walz had concluded that a reassuring word from an adult could make a huge impact, Mr Reitan recalled her telling him.

She and her husband had grown close to a gay student years earlier in Nebraska, where the Walzes met each other teaching at a public school. The couple took the student to an Indigo Girls concert, a spokeswoman for Ms. Walz said, the rare queer-friendly event in that time.

For Ms. Walz, being an ally for gay students was a matter of living up to the tenets of her Christian faith, Claire Lancaster, the spokeswoman, said, adding that Ms. Walz has alluded to a “strong belief that God creates people in the way they are supposed to be, whether that is gay or straight.”

Mr. Reitan said that he thought long and hard before he began sharing his secret. He first said those words out loud, he recalled, to his family dog, a golden shepherd named Alex.

During his junior year, he told his older sister. That led him to confide in Ms. Walz, the one adult at school he felt certain would support him. Her warm response, he said, gave him the confidence to confide in his parents, Randi and Phil Reitan.

“It wasn’t easy for us because immediately I felt like he was going to face a lifetime of discrimination,” Ms. Reitan, 73, said in an interview.

The Reitans sought guidance from a trusted pastor but said that his response was crushing: Their son was living in sin; turning to God could make him straight. They said they then turned to an openly gay psychologist, who encouraged Mr. Reitan to refrain from coming out until college.

Jacob Reitan said that he and his parents argued over what to do when he faced an uptick in bullying during his final year in high school. The teenager was inclined to come out. His parents disagreed, in part because months earlier they had been horrified by news of the killing of Matthew Shepard, a gay college student who was kidnapped, pistol-whipped and tied to a fence in Wyoming in October 1998.

Mr. Shepard resembled her son, Ms. Reitan said, adding: “That weighed on my heart, hugely.”

Ultimately, Mr. Reitan was adamant that he would come out before receiving his high school diploma. It was important for his classmates to get to know an openly gay person, he reasoned.

He came out to classmates in the summer of 1999 and later that year invited peers to join the gay-straight alliance club by passing out leaflets at the front entrance of the school.

Mr. Walz has made clear that he understood why he had been picked as the adviser. “You have an older, white, straight, married football coach who is deeply concerned that these students are treated fairly and that there’s no bullying,” Mr. Walz said while he was running for governor, as he drew attention to his involvement in the club years before.

Club meetings drew a smattering of students. No one else came out of the closet during the period Mr. Reitan led it, he said. But the bullying largely ceased.

“The Walzes were both like, ‘There will be nothing but respect,’ and it was just like laying down the law,” recalled Nicole Griensewic, 41, a former classmate of Mr. Reitan’s. “It was really bold.”

Years later, when Randi Reitan learned that Mr. Walz was running for Congress, she raised money and campaigned for her son’s former protector. At the time, the battle to legalize same-sex marriage was being waged in courtrooms and statehouses — facing long odds.

Ms. Reitan recalled urging Mr. Walz not to take a position on the matter. His candidacy was a long shot in a rural district. “We just want you to get elected,” she recalled telling him.

But Ms. Reitan said that Mr. Walz pushed back. “This is who I am,” she recalled him saying. “I want to be authentic and I want to be able to look my gay students in the eye.”