Three dozen Catholics from “Lebo,” as the Mt. Lebanon area of Pittsburgh is called, packed into Colleen Oxenreiter’s living room on a recent Friday evening. Lebo is a heavily Catholic neighborhood, and her home was the one with a giant Trump sign in the yard, by her pumpkins.

She explained the group’s mission: to reach out, in a campaign of postcards and video text messages, to Catholic Republicans who did not vote in 2020. If they vote this time, she hoped, it could be enough to win back the battleground state, and the White House.

The group had another big motivator: JD Vance, the 40-year-old Republican vice-presidential nominee.

Ms. Oxenreiter recounted how a friend had recently raised concerns that former President Donald Trump was “too old.”

“I said, ‘Well, that’s why he picked Vance!’” she said. “Another eight years!”

With Election Day closing in, white Catholic voters could prove important to the G.O.P. in Pennsylvania, a swing state often won by razor-thin margins. White Catholics tend to be reliably Republican, but in 2020, Mr. Biden, a Catholic born in Pennsylvania, capitalized on his cultural affinity with them, at least enough to narrow the partisan gap, and won the state.

In 2024, Mr. Vance is the only Catholic candidate on either ticket, a convert, economic populist and outspoken social conservative. His political profile resonates with many of these voters, who are worried about issues like education, transgender rights and the economy, in addition to abortion.