Pennsylvania voters can at last weigh debate performance in their choice for senator. When they do, they’ll recall seeing Democrat John Fetterman self-destruct with a belligerent display, snatching defeat from the jaws of victory against Republican Dr. Mehmet Oz.

Filmmaker Woody Allen once said, “Eighty percent of success is showing up,” and because of complications from a stroke, Mr. Fetterman could have triumphed just by stepping onto the stage and running out the clock, winning by not losing, maintaining the status quo where he maintains a lead in the FiveThirtyEight polling average.

That’s how he came out of the gate. Declaring common cause with all those who have been given a tumble by life, he all but sang Chumbawamba’s 1997 hit “Tubthumping” with its memorable lyrics, “I get knocked down, but I get up again. You’re never gonna keep me down.”

It was feel-good politics that evoked sympathy, until he swerved into insulting Dr. Oz. It was a sucker punch to undecided and Republican voters, one that politicized his illness. If Mr. Fetterman sought to demonstrate that he could still be a fierce advocate, he failed.

The ensuing performance made him appear like a dangerous, unhinged rider on the Philadelphia SEPTA — the kind that makes women clutch their purses, men put their phones away to have their fists free, and people of all sorts get off at the next stop.

At 6’3”, I’ve seen how my size and shouting can unsettle strangers. Mr. Fetterman — almost half a foot taller than I am — either doesn’t realize that when angry, he comes across like Bruce Banner about to unleash The Hulk or is unable to hold his temper due to neurological damage.

For example, he shouted each time he said, “Roe v. Wade,” and after his opponent was polite and allowed him to complete a closing statement, Mr. Fetterman interrupted Dr. Oz’s last word with an angry, “You want to cut Social Security!”

Viewers had just watched Dr. Oz ask Mr. Fetterman where he had said any such thing; so, the outburst did double hit points of damage to the giant, reminding viewers that he hadn’t backed up his charge, any more than he had his claim of distance from Senator Sanders, who he boasted of endorsing in 2016 because “he and I agree on virtually every issue.”

Mr. Fetterman gave the impression of being programmed and of trying too hard, never more so than when he pushed — again and again — to make “The Oz Rule” catch on as a synonym for lying, all but begging voters to hashtag it.

Meanwhile, at the other podium, Dr. Oz looked as comfortable in front of a TV camera as he might in his living room. In his opening, he didn’t attack Mr. Fetterman, focusing instead on inflation, crime, and the cost of food, cutting up his opponent with a surgeon’s skill.

The only time he came close to nicking an artery was when he said, “John, I wasn’t clear enough for you to understand,” and even then, he took the blame, like you’d expect of a compassionate physician.

Speaking of the Republican’s profession, the moderators referring to him as “Mr. Oz” sounded off to ears conditioned over two decades of television appearances. It gave the impression of disrespect, and each time he ignored it, Dr. Oz underlined his unflappability.

This debate occurred so late that 500,000 votes have already been cast according to the Pennsylvania Department of State and millions in ads have ensured that everyone knows where the candidates stand, which limited the importance of issues. This was a test drive of the only two cars for sale on the lot.

Mr. Fetterman just needed to reassure voters that his motor could get them around town without stalling. Instead, he put the pedal to the metal and swerved off the road. It’s this inability to execute, not the ravages of his stroke, that will endure.

The Democrat got knocked down in Harrisburg. Whether he gets up again is now in the hands of the people, and partisans trying to spin his self-destruction as a win.