Mr. Mastriano speaks almost exclusively to far-right podcasters like Stephen K. Bannon, conservative talk radio hosts and Fox News. On a recent swing through northwest Pennsylvania, he brushed off a Pittsburgh TV station that sought to interview him, and even the small-circulation Meadville Tribune.

One result of that approach is that he seldom has to field tough questions. And his poor fund-raising — he ended the primary season with just $400,000 in his campaign account, compared with $13.4 million for Mr. Shapiro — has left him unable to run TV ads all summer to counter a barrage of attacks from his opponent.

The Shapiro ads use Mr. Mastriano’s words to paint him as outside the mainstream, not just on abortion and election denial, but on gay marriage, which he has said should “absolutely not” be legal, and on global warming, which he called “fake science.”

“You’ve basically got a one-person governor’s race right now in terms of voter contact,” said Christopher Nicholas, a Republican consultant in the state. “All the folks who listen to those far-right podcasts, I think he maxed out his vote potential. He has to move past his base.”