As partisans spin the debate performances by Presidents Trump and Biden, the candidates are left reviewing the tape with an eye to the future. The CNN slugfest offers voters the choice of a politician in Mr. Biden who follows a traditional strategy and an unconventional challenger who relies on brute force and instinct.

By skipping the Republican primary debates, Trump gift-wrapped an excuse for Mr. Biden to avoid last night. Indeed, he’d been playing coy, offering cryptic responses such as he’d meet Trump “if he behaves.” That he sought this faceoff demonstrated that his campaign is on the back foot. He took the risk, and it didn’t pay.

CNN’s national correspondent, Johnathan King, said, “there is a deep, a wide, and a very aggressive panic in the Democratic Party” following Mr. Biden’s performance. It had, Mr. King said, sparked conversations about asking “the president to step aside.”

Although Mr. King said Democrats he spoke to agreed that Mr. Biden “got better and got his footing” as the 90-minute forum wore on, they noted that “even his closing statement was a little halting.” They felt, Mr. King said, that “this debate was so terrible.”

The chief strategist for President Obama, David Axelrod, said on CNN that Mr. Biden’s performance “helped confirm the fear” that he lacks the mental and physical stamina for the job. He predicted “you’re going to hear discussions … about whether he should continue.”

In 1973’s “Magnum Force,” Dirty Harry, played by Clint Eastwood, says, “A man’s got to know his limitations.” After last night, Mr. Biden knows his. Going forward, he might take the tack of President McKinley when he ran against the famed orator, Congressman William Jennings Bryan, in 1896.

Bryan, known as the Great Commoner, had a skill McKinley couldn’t hope to match; so, he didn’t try. “I might just as well put up a trapeze on my front lawn and compete with some professional athlete,” he said, “as go out speaking against Bryan.”

Mr. Biden’s week huddled with advisors at Camp David failed to turn him into an acrobat with the Flying Graysons. Like President Reagan, who lamented over preparing for his first debate against Vice President Mondale in 1984, Mr. Biden sputtered out of the gate, tripping over his words to deliver canned rhetoric.

Reagan rebounded in the second faceoff, offering his famous deadpan about not using Mondale’s “youth and inexperience” against him. Mr. Biden rallied in the latter rounds, but his campaign helped cement the bad first impression well before the final bell.

“He has a cold,” a source “familiar” with Mr. Biden’s campaign told the Hill during the debate. It seems premature and defensive now given how the president rallied. It’s spin that, in hindsight, might have worked better if slipped to friends in the press later.

The weight we put on physical performance in presidential debates comes from an enduring myth dating back to the first one in 1960. It holds that those who listened on the radio judged President Nixon the winner while TV viewers favored President Kennedy. In fact, no such gulf existed.

Mr. Biden can hope the same holds true for him. Although on the split screen at times he seemed vacant, mouth agape, when Mr. Trump spoke, his hottest moments are what will be replayed in the press. The bruises he suffered are fresh now, but the news cycle has a way of healing wounds.

Trump also has work to do to solidify his gains. He looked relaxed and in command, smiling straight into the camera, his ruddy complexion a contrast to the pallor of Mr. Biden. Democrats’ hopes that the Republican would self-destruct when confronted with his felony convictions came to nothing.

In a counterpunch anyone might have seen telegraphed a mile away, Trump responded that Mr. Biden’s son, Hunter, “is a convicted felon,” too. He focused on inflation, illegal immigration, jobs, taxes, and other issues, offering the electorate perspectives from the heart while his opponent aimed to get inside their heads.

Moving to favorite topics — legislation and foreign policy — rejuvenated Mr. Biden, and he tore into Trump’s record. He repeated the dubious story that Trump referred to military dead as “suckers and losers” and the debunked one about Trump calling neo-Nazis at Charlottesville “very fine people.”

This might’ve been hitting below the belt, but rather than attack the moderators for not calling fouls as opponents expected, Trump mounted calm defenses. Given the chance, he counterattacked with exaggerated claims of his own.  Both men left it to voters to sort out the truth.

Throughout the night, Mr. Biden landed blows on policy while Trump deferred opportunities to respond in kind. On abortion, Trump might have mentioned that Mr. Biden, as a senator in 1982, voted to advance a bill overturning Roe v. Wade. It’s the kind of haymaker a traditional candidate would have been itching to unleash.

Rather than aiming for targets picked in advance, Trump laid back and waited for openings. Asked to respond to one rambling answer by Mr. Biden, he landed a one-two punch. “I really don’t know what he said at the end of that sentence,” he said. “I don’t think he knows what he said either.”

Thanks to CNN muting the candidates’ microphones when they weren’t speaking, no response from Mr. Biden could have been heard to this barb, but he’d have been helped if he’d at least mouthed something. Instead, obeying the Marquess of Queensberry rules both candidates agreed to, he stood silent.

“Everybody has a plan,” the former heavyweight champion, Mike Tyson, said, “until they get punched in the face.” Trump applied that philosophy last night, but a single debate can’t win or lose the White House. Now Mr. Biden will assess his limitations and see if he can answer the bell when the candidates next meet in September.