LAUGHLINTOWN, Pa. — Michael Testa, 51, an Army veteran and handyman, drives a minivan plastered with stickers reading “Trump Won.”

He recently stood in the rain and mud for hours to attend Donald Trump’s Pennsylvania rally. He calls himself a “conspiracy realist” and said he’s one of millions who believe the 2020 election was stolen from the former president.

But as he sat on his front porch in Laughlintown, a small borough of Westmoreland County outside Pittsburgh that was once home to the Mellon family fortune, he was undecided about which candidate to vote for on Tuesday in Pennsylvania’s Republican primary for Senate. He has misgivings about supporting Mehmet Oz, the celebrity doctor Mr. Trump has endorsed.

“I’m not going to be somebody who does something just because one person says so, even if that person is Trump,” Mr. Testa said.

Like other Republican primaries throughout the country, the Pennsylvania Senate race is testing just how strong Mr. Trump’s grip remains on the party. But unlike other primaries this year, the Senate contest in Pennsylvania has suddenly pivoted into something else — a case study of whether the movement Mr. Trump created remains within his control.

In interviews with more than two dozen Republican voters in western Pennsylvania, many echoed Mr. Testa’s ambivalence and uncertainty about Dr. Oz — despite Mr. Trump’s backing, they view him with suspicion, call him “too Hollywood” and question his ties to the state. Those Republicans, including Mr. Testa, said they were instead voting for or considering voting for Kathy Barnette, the far-right author and conservative-media commentator who has surged in the polls on a shoestring budget.

In a race that could determine control of the Senate, many Republicans in the state find themselves deeply devoted to Mr. Trump yet, at the same time, less swayed by his guidance. Trumpism, as Ms. Barnette herself has put it on the campaign trail, is bigger than Trump.

Many voters said they were choosing who they believed would carry out Mr. Trump’s ideals, even if they and the former president disagreed on who could best accomplish that. And interviews showed how effectively Ms. Barnette, who has never held public office, had used her life story as a poor, Black child of the South to connect with white working-class voters in western Pennsylvania. At events and in her ads, Ms. Barnette often invokes the phrase “I am you.”

Many voters who said they planned to vote for Ms. Barnette struggled to remember her name and said they were supporting “that Black woman.” Those who said they were voting for her said they were unaware of or unbothered by her history of homophobic and anti-Muslim views. But her strong anti-abortion beliefs — Ms. Barnette calls herself a “byproduct of rape”— have been a key part of her appeal to white conservatives.

“I like what she stands for,” said Dolores Mrozinski, 83, who first watched Ms. Barnette on the Christian Television Network and was immediately impressed. “She’s no-nonsense and the real thing.”

Years ago, Ms. Mrozinski and her daughter, Janey Mrozinski, a 62-year-old physical therapist, watched Dr. Oz on television and even admired him. Now, the elder Ms. Mrozinski said, “he just doesn’t seem genuine.”

“I don’t even know if he really lives in Pennsylvania,” she said, referring to Dr. Oz’s long history, until recent years, of living and voting in New Jersey. “He seems more Hollywood than here and it doesn’t impress me.”

Her daughter added, “He looks like he had a face lift.” On the other hand, David McCormick, a former hedge fund executive who is also running in the primary, was simply, she said, “too much, too proud of himself.”

In many ways, the vote for the Senate seat is as much a battle over the perception of authenticity as any ideological or policy debate. For months now, the leading candidates have each tried to align themselves closely with Mr. Trump and promote their conservative credentials. In the tight contest between the leading contenders — Dr. Oz, Ms. Barnette and Mr. McCormick — all three of them have tried hard to cast themselves as the true MAGA warrior.

Some voters have clearly made up their minds about which one they believe is more authentic. But others are still deciding.

One glance at John Artzberger’s auto body shop along Highway 8 in Butler County makes his political leanings clear: A “Let’s Go Brandon” flag flies from the shop’s marquee, and Trump paraphernalia covers a large wall near the entrance. When one customer asked him to place a Barnette lawn sign out front, he did not hesitate to agree. Still, the sign was just a sign — he said he was undecided and considering voting for either Ms. Barnette or Dr. Oz.

“She’s 100 percent on our side — close the border, pro-life,” Mr. Artzberger, 68, said of Ms. Barnette. “If she gets it, she’s going to be for the people.” Like many other Republicans in Butler County, Mr. Artzberger views Dr. Oz’s previous time in the spotlight with disdain.

“But then again, Trump had been in the public eye, too, and he ended up being really with us,” he said. “I’ve changed, so maybe he changed, too.”

In Laughlintown in Westmoreland County, it takes about 10 steps to travel from the front porch of Mr. Testa’s old Craftsman to the front doors of the small brick church next door. In that short distance lies a glimpse of the Republican Party’s identity crisis.

Jonathan Huddleston, 48, the minister of Laughlintown Christian Church, calls himself a Never-Trump Republican but remains committed to the party to, in part, “help vote the wackos out.” He, too, is undecided — he is considering voting for Mr. McCormick, who tried but failed to win the Trump endorsement.

“I want to support the Romneys of the world, the reasonable leaders, the ones who drew me to begin with,” Mr. Huddleston said. “Now I’m searching to find people like that. All of the other voices are drowning them out.”

Some Republican voters said they had tried to tune out the deluge of attack ads on television from Mr. McCormick and Dr. Oz, who have each spent millions of dollars of their own wealth in the race. The backlash against the Oz and McCormick ads appeared to benefit Ms. Barnette, who has spent less than $200,000 in her campaign.

“It’s just every moment and nothing about what they say they’re going to do or how they’re going to help people,” said Jeannie Gsell, 70, who lives in Greensburg, about 30 miles east of Pittsburgh.

In 2020, Ms. Gsell, a registered Republican, voted for President Biden, after some convincing from her liberal daughter. But she said she had been disappointed by his time in the White House. She plans to vote in Tuesday’s Republican primary but is still undecided. She said she would make up her mind by deciding whom she finds most sincere.

“People should be going to Washington to take care of regular people’s priorities, not taking care of themselves and getting more rich or more famous,” Ms. Gsell said.

In downtown Butler, a working-class city north of Pittsburgh, Brittney Meehan, a 34-year-old waitress, said the two most important issues for her were “guns and weed — two that don’t usually go together.”

Ms. Meehan said she was “not absolutely sold on voting Republican,” citing her commitment to supporting both gun rights and abortion rights. “What I want is a real person, not people who are up on that level, but are just in touch as human beings,” she added.

Ms. Meehan said she wished “people would just hear each other out when they disagree,” a sentiment shared by Mr. Huddleston, the minister in Laughlintown.

“I want to have honesty and respect, is that really so impossible now?” Mr. Huddleston said as he sat in the church pews one recent afternoon.

He thinks about voters like his next-door neighbor Mr. Testa and wonders what will become of moderate Republicans like himself. The two men know each other, but they haven’t spoken about politics directly. He noticed his neighbor’s many bumper stickers. One of them reads, “I pledged an oath to protect against foreign and domestic.” He wondered about the meaning. For now, though, he said, “I haven’t felt it was the neighborly thing to do to ask.”