“Many voters respond to style more than substance. The well-delivered quip lingers longer than the litany of facts, and the visual often trumps the verbal”. 

What most of us instinctively know, clarified here by a long-term observer of US election debates. Analyzing a debate point by point and fact-checking are useful but do not gauge the reactions of the voters.

If you’re a typical American voter in any party, allow me to let you in on a little secret: What matters most to you in a presidential debate probably isn’t the same thing that gets the most attention from the candidates, the campaigns and their allies in the immediate aftermath of those big televised showdowns.

I’ve learned this from studying American reactions to almost every general election presidential debate since 1992. I’ve sat with small groups of voters selected from pools of thousands of undecided voters nationally, watching more than two dozen presidential and vice-presidential debates in real time, and it still amazes me that minuscule moments, verbal miscues and misremembering little details can matter so much in the spin room and to partisan pundits afterward. Yet those things often have little to no discernible impact on the opinions of many people watching at home.

To be fair, some of the debates I watched with voters, like Bill Clinton and Bob Dole’s in 1996, had no major impact on the electorate’s mood. Others — like the three-way town hall debate with Mr. Clinton, George H.W. Bush and Ross Perot in 1992 and the first George W. Bush-Al Gore debate in 2000 and the three Donald Trump-Hillary Clinton collisions — arguably changed history.

As the first scheduled debate between President Biden and Mr. Trump unfolds this Thursday, the key moments that will have the greatest impact on the remaining undecided voters are those in which the candidates attack each other in defining ways or undermine the political case that each wants to present to Americans. Viewers will quickly decide whether the accusations are fair and the responses effective. From Ronald Reagan’s “Are you better off than you were four years ago?” in 1980 to Barack Obama emphasizing hope and change in 2008 to Mr. Trump telling Mrs. Clinton in 2016 that she would “be in jail” if he won, I think those key debate moments made a meaningful difference in shaping the opinions of undecided or wavering voters who related to what they heard; I certainly saw it in my focus groups and public opinion research. These moments mattered more than any candidate flub or gaffe.

And sometimes it’s a feeling rather than a specific moment that matters. The best examples are John Kerry in the 2004 debates and John McCain in the 2008 debates: Both men were good public servants with impressive personal narratives, and neither said anything wrong in their debates. But neither did they say anything especially or memorably right. Many voters were left feeling unmoved and therefore unaffected.

At the risk of offending every high school debate coach in America, many voters respond to style more than substance. The well-delivered quip lingers longer than the litany of facts, and the visual often trumps the verbal. It’s not just that the electorate tends to be drawn more to younger and more attractive candidates (like Mr. Obama, Mr. Clinton and John F. Kennedy) or to those with more commanding stage presence (which Mr. Reagan had over Jimmy Carter and Walter Mondale, and George H.W. Bush had over Michael Dukakis). While the 2016 and 2020 debates featuring Mr. Trump certainly upended our collective expectations about what exactly is presidential, listening to the voters describe each debate and their gut impressions of the candidates is more instructive about the eventual election winner than getting swept up in spin and punditry.

Perhaps the single best example of divergence between voter opinions and the views of politicos and pundits was the 1992 town hall debate. In the immediate aftermath, Mr. Bush was pilloried by the professional class for checking his watch during the debate — a moment that was completely missed by my focus group of American voters. To them, the biggest takeaways were Mr. Bush’s inability to explain what the federal deficit meant to him and then Mr. Clinton’s Oscar-worthy performance as he deftly stood up from his stool and approached an audience member with empathy and compassion, her head nodding in agreement with him throughout the encounter.

A similar misreading of a debate performance came from the first debate between George W. Bush and Mr. Gore, when a number of political analysts praised Mr. Gore for his command of the facts and intricacies of presidential decision making, while much of America seemed pleasantly surprised (shocked, actually) that Mr. Bush was able to string together complete sentences that were competent, coherent and compelling. Voters in my focus group were impressed with Mr. Bush’s comfort and command of the debate stage and disappointed with Mr. Gore’s stiffness and annoyed with what they saw as his dismissiveness toward his opponent.

In almost every presidential debate since 1992, voter expectations of a candidate’s performance also played a major role in determining perceptions of success and failure. Many had low expectations of Mr. Bush in 2000 and Mr. Trump in 2016 (and Mr. Biden now). The fact that they didn’t completely flop led at least some voters to see these candidates as surprisingly successful in the debates.

Many election observers believe that the incumbents start with some advantage because they have national debate experience and a command of governing. In Thursday’s case, both men have that experience, so voters will be looking at other factors — probably related to energy, sharpness and how they come across. While the specific circumstances were different, I think about the shock I felt watching Mr. Obama and Mitt Romney in their first debate in 2012. The widely held assumption was that Mr. Obama’s grace and charm would easily overwhelm Mr. Romney’s stiff and businesslike approach. But Mr. Obama was so chill in his approach that he came across as cold and uncaring to many voters. His performance was criticized by my focus group for lacking his customary passion and conviction — a surprising evaluation from a politician so popular for those qualities.

But here’s the surprising twist: In time, many voters came to see that first encounter with more nuance than that instant reaction suggested. In my Election Day 2012 focus groups, voters said they were left thinking that Mr. Obama truly understood them and their concerns but that he had no answers or solutions to their problems. Conversely, they felt that Mr. Romney had the better solutions to the challenges they faced but that he just didn’t fully understand their problems. Yes, policy solutions definitely matter in presidential debates. But personality, relatability and dignity matter more.

And it’s not just the candidate’s personal performance that leaves an impression. Sometimes forces that are less visible, like the debate rules, play a major role in determining the outcome. The length of time given to respond to questions from the moderator can reward or punish candidates, depending on their individual styles and ability to communicate succinctly. Nothing draws the ire of the average voter more than candidates speaking beyond their allotted time, my focus groups have shown. While most professional debate observers ignore candidates who run long, voters punish them mercilessly. It was a major reason many undecided voters turned so strongly against Mr. Trump after his undisciplined performance in the first debate in 2020.

That debate, the most consequential one in memory, was one in which many voters and political experts drew roughly the same conclusions. Mr. Trump entered the debate trailing Mr. Biden by just a couple of percentage points, but his questionable strategy to insult, badger and bully Mr. Biden was received so badly by the women in my focus group that they were as harsh about Mr. Trump as he was to Mr. Biden.

In contrast, there was one moment in the Trump-Clinton debates when voter opinion really struck me. It was Mr. Trump’s offhand comment that Mrs. Clinton belonged in jail. Many pundits and political experts hated it. My focus group loved it. For them, it was accountability in action for someone as important as her, a former secretary of state. To be sure, many political experts zeroed in on the moment as a striking instance of a presidential nominee threatening to weaponize the justice system against his opponent. But I think what they missed was a yearning among some voters to see a senior official held to account and not let off the hook by a system seen as protecting insiders.

This week brings us potentially one of the most consequential debates since Mr. Kennedy and Richard Nixon’s. The expectations are already high for Mr. Trump, who dared Mr. Biden to debate at any time or place of his choosing. It is quite possible that Mr. Trump will regret issuing such a public challenge, and Mr. Biden may regret accepting the offer.

To shape and sway voter opinion, the two opponents need to use the debate to do what Mr. Reagan, Mr. Obama and Mr. Trump did at their best: Crystallize the stakes of the race and the choice in November with a single memorable line that speaks to the feelings, instincts and perhaps even the fears of so many voters about America today.

Given that viewers are conditioned to see the 2024 debates as a mix of television entertainment and a war for America’s future, they will want to see passion, energy and even anger in service to the interests of the country. A self-controlled Mr. Trump or an adult Mr. Biden won’t be remembered, just as Mr. Kerry and Mr. McCain weren’t remembered. So much is at stake that both candidates need to let loose to make a lasting impression but not in a way that may alienate key groups like suburban women and swing voters.

In the end, it’s not the facts, the policies or even the one-upmanship that Mr. Biden and Mr. Trump offer in the debate that matters. It’s how they make voters feel.