What was left for the fight over the Senate seat deemed most within reach for a Democratic takeover was a heart patient, Mr. Fetterman, battling a heart surgeon, Dr. Oz, and a distinct sense of unease, at least for now, among some Democrats.

“It’s going to be a brutal campaign,” said G. Terry Madonna, a longtime pollster and political writer in Pennsylvania. “If I had to put it in a couple of words, it’s ‘no holds barred.’”

Mr. Fetterman’s health struggles could be particularly resonant because so much of his appeal has stemmed from his image of vitality. Though he hails from his party’s left flank, he has garnered the affections of more moderate, working-class voters with his bald head, goateed face, Carhartt sweatshirts, baggy basketball shorts and tireless campaigning in every nook of the state…

Now that common-man appeal must include the contrition of an ailing patient who ignored his doctor’s advice for years and an overt appeal to the sympathies of the voters.