“It’s a throwback to a phenomenon that has essentially disappeared in the era of digital media,” said Andrew Heyward, a former president of CBS News. “This is Americans gathering around the electronic hearth.”

Advertisers, including Audi cars and Tecate beer, are taking advantage, introducing debate-themed commercials in the kind of tie-in marketing usually reserved for events like the Super Bowl. Although the debate will air without commercials, cable and broadcast channels have sold millions of dollars’ worth of ads for programming before and afterward.

Television networks and online streaming sites, including Facebook and Twitter, will carry the same feed on Monday, showing a spare debate stage at Hofstra University, on Long Island, a format that predates the blaring graphics and space-age sets that now dominate television news.

Still, even if a large portion of the country is watching, what Americans see may be as much about their beliefs and preferred news outlets as what transpires onstage.

About 8 percent of registered voters remain undecided, according to the New York Times/CBS News poll, a thin if crucial sliver of the electorate. And after Mrs. Clinton and Mr. Trump conclude on Monday, viewers are likely to return to their ideological silos, absorbing instant analysis from left-leaning anchors on MSNBC or commentators at right-leaning outlets like Breitbart News.

The debate itself will be subject to instant, blow-by-blow interpretation on social media.

“Regardless of where you’re watching, whether it’s Facebook Live or NBC or Fox News, there will be a moment where we all witness it,” said Charles L. Ponce de Leon, author of “That’s the Way It Is,” a history of television news. “But that moment will quickly crumble when all the instant analysis and opining comes into play.”

The event’s impact is unlikely to rival that of, say, 1960, when John F. Kennedy’s smooth performance in the first televised debate helped sway voters against his opponent, Richard Nixon. That debate aired without commentary — or graphics and captions on the screen. “Journalists were of the opinion they should wait and ruminate and think about what went down, and then, a day or a week later, talk about it,” Mr. de Leon said.

Tom Sander, who runs a program on civic engagement at the Harvard Kennedy School of Government, said he was worried that voters might not have a chance to remove partisan blinders.

“Many of us come to these events more to confirm what we already think we know, rather than to search for common ground,” Mr. Sander said.

The candidates may hope otherwise. Mrs. Clinton has told donors privately that she expects 100 million people to watch the debate, and that 60 million of those viewers may be focusing on the campaign for the first time, a prime opportunity for her to make inroads.

Many may tune in merely for the spectacle.

“It’s like waiting for the Ali-Frazier fight,” said Dick Cavett, the longtime talk-show host, referring to the highly anticipated boxing bouts between Muhammad Ali and Joe Frazier in the 1970s. Mr. Cavett said he would cut short a dinner on Monday to ensure he would be in front of a TV by 9 p.m.

“There’s possible drama and fireworks and insults and horror and disaster and potential enlightenment,” Mr. Cavett said. “It would attract anybody.”

Some in TV noted that the 2008 meeting between Joseph R. Biden and Sarah Palin attracted 70 million viewers, more than any of that year’s presidential debates, a sign that civic interest may be less of a draw than seeing a colorful candidate like Ms. Palin — or Mr. Trump.

The 1980 Carter-Reagan debate scored a record audience in part because it was the only head-to-head matchup between the candidates in a precable era. But Neal Shapiro, a former president of NBC News, recalled that Mr. Reagan’s unusual background as a Hollywood actor spurred interest.

“People wanted to see, would they really feel comfortable with Reagan as president?” Mr. Shapiro said. “People were wondering, ‘Can I live with this guy?’”

The first debate between President Obama and Mitt Romney in 2012 drew 67 million viewers. Advertisers, anticipating even more this time, are using the opportunity to unveil new ads with themes that may resonate with Americans focused on the campaign.

Audi’s commercial shows a man and woman, both hotel valets, battling for the right to drive an Audi RS7 luxury car, and features the slogan “Choose the next driver wisely.” American flags, and ice sculptures of a donkey and an elephant, convey the message that the car, as an Audi vice president for marketing put it, is “a metaphor for the importance of America.”