Michael Korns, chairman of the Republican committee here in Westmoreland County, is confident that Mr. Trump can win Pennsylvania, where Mr. Obama defeated Mitt Romney in 2012 by a margin of 310,000 votes, 52 percent to 46.6 percent.

“Many voters feel that the Democratic Party, which they had supported for generations, has largely abandoned blue-collar workers,” Mr. Korns said. “There’s also increasingly a feeling that the Republican Party has abandoned them as well, that neither party has much interest in the day-to-day economics of working people. And then when Trump came in, he spoke to them, he grabbed them.”

G. Terry Madonna, a professor of public affairs at Franklin & Marshall College in Lancaster, said Mr. Trump might do considerably better than Mr. Romney, who lost every Rust Belt state except Indiana.

“It’s a combination of the frustration that a lot of Americans feel — white, blue-collar workers who believe the economy is passing them by, that there’s still a recession, that wages haven’t kept pace,” Mr. Madonna said. “There’s all this frustration, and then a master showman shows up who says he’ll stick it to the establishment.”