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.