Mass experiences — built around news events like the moon landing, and pop culture moments for older generations like the “Who shot J. R.?” episode of “Dallas” — are rare in an age of fragmented media and the drift toward partisan outlets, where viewers can effectively choose their own news.

But tight polls and curiosity about the unconventional Mr. Trump are luring viewers. In a New York Times/CBS News poll this month, 83 percent of registered voters said they were very or somewhat likely to watch on Monday....

“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.