Even now, after a community panic and a rush to legislate in the state house in Boise, people in Sandpoint, Idaho, aren’t certain what actually happened last year in a locker room at the local Y.M.C.A.

Two women, one transgender, one who identifies with her biological sex, interacted for a few moments on an otherwise ordinary day. Then the encounter took on a life of its own.

“I’ve heard people call it an ‘incident,’” said Zach Hagadone, editor of the Sandpoint Reader, a local newspaper. “It wasn’t an incident. It was a nonevent that certain people used for their own purposes.”

With Sandpoint as their rallying cry, Idaho lawmakers passed the nation’s toughest bathroom law this spring, making it a crime for a person to use a facility that does not match their assigned sex. But the Y.M.C.A. moment in Sandpoint has also shown how some conservative groups hope to use transgender women in women’s bathrooms to roll back a far broader set of civil rights protections.

“We want to prevent all the bizarreness and keep our state normal,” said Bruce Skaug, a Republican Idaho state representative from Nampa, near Boise, who is sponsoring legislation to bar cities and counties from adopting local nondiscrimination policies that extend protection to sexual orientation or gender identity.

The remote part of the state’s panhandle, called “North Idaho” by locals, has drawn anti-government extremists for decades. The deadly 1992 Ruby Ridge standoff between federal agents and Randy Weaver took place 30 miles north of Sandpoint. A white-supremacist Aryan Nations compound was 40 miles south until a lawsuit bankrupted organizers in 2001. And in the early 2010s, the region became a hub for political and religious conservatives answering the survivalist author James Wesley Rawles’s call for an “American Redoubt.”

The remnants of a different culture — tie-dye, a booming arts scene and a live-and-let-live mentality — can also be found in Sandpoint, signs of the hippies who came in the 1960s and ’70s to seek their own haven. A Ten Commandments monument anchors a downtown park, steps away from a Pride flag overlaid with the Gadsden rattlesnake and “Don’t Tread on Me” logo.

“Even when I was a kid in the school system, they always just sort of knew I wasn’t the marrying kind,” said Doug Jones, a 70-year-old graphic designer and artist who grew up gay in Sandpoint. “But there was this acceptance built on personal connection.”

The City Council passed the state’s first local nondiscrimination ordinance in 2011, inspiring a dozen other Idaho communities to adopt similar policies.

“We didn’t have any state laws that protected people based on their gender identity or sexual orientation, so you could get married, put a picture of your spouse on your desk, and be fired for it the next day,” said John Reuter, council president at the time.

Over the past 15 years, Bonner County grew to nearly 55,000 residents from 40,000 as conservative political refugees from bluer West Coast states discovered a land of fresh air and floatplanes. McMansions have replaced fishing shacks on Lake Pend Oreille and dot the hillsides around Schweitzer Mountain Resort.

The politics, as in most of Idaho, has shifted from red to deep red, because newcomers find Sandpoint “a little more normal than they expect, so they try to change it,” said Jim Woodward, the area’s state senator.

Mr. Woodward, a Republican, has traded the legislative seat multiple times in recent years with Scott Herndon, a hard-right Republican whose platform included ending mandatory vaccinations for schoolchildren and stopping “the transgender nonsense.” Mr. Herndon defeated Mr. Woodward in this year’s primary after campaigning heavily on the locker room moment, which the incumbent dismissed as “a nonevent turned into part of an ideological battle.”

That incident unfolded on Oct. 15, when Jennifer Hook, a staff member at the Litehouse Y.M.C.A., saw a transgender woman leaving a women’s locker room shower in a bra and towel. The transgender woman was familiar to other gym employees from previous visits, but not to Ms. Hook.

Ms. Hook could not be reached by phone and did not respond to a message left at her home outside Sandpoint or to a message to her Facebook account.

At the time, she wrote on social media, “I did not see his genitalia. But I could tell that this was a man.”

Ms. Hook asked her bosses to call 911, according to a later Facebook post. When they told her the transgender woman wasn’t violating any law or policy, Ms. Hook called the police herself. The responding officer told her there was nothing he could do and that she should contact the Legislature if she wanted the law changed.

Ms. Hook’s story spread on Facebook, along with expressions of fear that a man dressed in women’s clothing could slip into a women’s locker room to prey on girls. It also prompted calls, from Mr. Herndon and others on the right, for city leaders to do something.

Five weeks later, Mayor Jeremy Grimm asked the City Council to repeal Sandpoint’s nondiscrimination ordinance. At a meeting that lasted almost five hours, dozens of people spoke. Most argued that removing any specific local protections was the only way to protect women and girls from men in public restrooms, and many derided transgender people as mentally ill.

Mr. Grimm said he cast the tiebreaking vote to honor the women expressing fear, including his wife, the first person to testify that night.

“Cities should be in the business of water, sewer, parks, police,” he said. “I thought the easiest solution was to get us back to basics.”

In retrospect, he said, “we maybe threw the baby out with the bath water” by repealing the ordinance rather than tweaking it. Still, he added, “it was swift, and then it was over, maybe not for someone who feels vulnerable now because of that, but it was generally over.”

Except it wasn’t. State lawmakers cited the Y.M.C.A. encounter and heard directly from Ms. Hook while passing legislation making it a crime for people to use bathrooms and changing rooms that do not correspond with their sex assigned at birth in most Idaho buildings open to the public, including restaurants, offices and stores. A federal judge put the law on hold, but Idaho’s attorney general has promised to fight for it.

Conservative lawmakers are moving to their next target, the 12 Idaho communities, including Boise, with their own nondiscrimination policies. The House passed a ban this spring, but it stalled in the Senate. Mr. Skaug plans to refile next year and is confident he has the votes for passage.

Mr. Skaug said he wants “to protect children” and follow through on the demands of his constituents, increasingly “angry conservatives” who have come to Idaho trying to escape liberalism.

Civil rights advocates say the goal of such legislation is to scare transgender and L.G.B.T.Q. residents out of Idaho. “At a minimum, they want people back into hiding,” said Andrea Marcoccio, a local community organizer and L.G.B.T.Q.-rights proponent who moved to Sandpoint in 2018 to open Matchwood Brewing Company with her husband.

The brewery is a progressive hub in a town with quiet economic and cultural segregation. People look for clues to social allegiance, such as the rainbow-painted dumpster outside Matchwood Brewery.

Matchwood hosted the town’s annual Pride festival last weekend, and Ms. Marcoccio tries to make the pub a safe space for families with queer children, such as Ashley Schobert.

Ashley, 15, came out as transgender to her parents last fall, telling them by phone while they were out shopping, and soon after starting hormone treatments in Washington state. Her doctor began by asking whether her family could leave Idaho.

Ashley came out to her teammates on the high school boys’ swim team in March, telling them she didn’t plan to try moving to the girls’ team. Teammates, she said, noted that she was “one of the good ones.”

Still, she thinks she may eventually need to use the family changing room at the Y.M.C.A., where the swim team practices.

“I’ve had to change next to guys with Nazi tattoos,” she said, “and that’s probably not going to be a safe environment for me.”