In the year’s first wave of New York Times/Siena College battleground polls back in May, the No. 1 predictor of whether someone had defected from Joe Biden was whether the respondent was Middle Eastern or North African — a sign of the political toll of the war in Gaza.

The No. 2 predictor? Whether the respondent had a very favorable view of the podcaster Joe Rogan.

Overall, Mr. Biden led Donald J. Trump by just 49 percent to 40 percent among Biden 2020 voters who said they had a very favorable view of Mr. Rogan. Those with a somewhat favorable view of Mr. Rogan were also relatively likely to defect, with Mr. Biden’s lead at just 75-18 percent. In contrast, Mr. Biden led, 88-6, among everyone else.

We haven’t asked about Mr. Rogan since Kamala Harris’s entry into the race. But the recent speculation that she will appear on his popular podcast before the election highlights that his fans and listeners represent one of the most important swing constituencies of the cycle. They’re also part of one of the cycle’s most important demographic trends: a possible Democratic collapse among young men.

Although polls have shown Democrats underperforming among voters age 18 to 29 all election cycle, it increasingly seems possible that most or perhaps all of that weakness is concentrated among young men.

Mr. Trump leads Ms. Harris among young men, 58 percent to 37 percent, across the last three Times/Siena national polls. Ms. Harris holds an even larger lead among young women, 67-28. Surprisingly, Ms. Harris is faring no better than Mr. Biden did among young men in the Times/Siena data, even as she’s made significant gains among young women.

If this enormous gender gap among young voters holds up, it will be one of the most important developments of the election season, one that would at once clarify a surprising trend and perhaps even change how we understand gender and cultural dynamics in America today.