Donald Trump leads him among those 18 to 29, a new poll shows.Palestine supporters in Washington on Sunday.Tasos Katopodis/Getty ImagesAs recently as this summer, a poll with Donald J. Trump leading among young voters would have been eye-popping.Now, it’s increasingly familiar — and our new New York Times/Siena College national survey released Tuesday morning is no exception.For the first time, Mr. Trump leads President Biden among young voters in a Times/Siena national survey, 49 percent to 43 percent. It’s enough to give him a narrow 46-44 lead among registered voters overall.Usually, it’s not worth dwelling too much on a subsample from a single poll, but this basic story about young voters is present in nearly every major survey at this point. Our own battleground state surveys in the fall showed something similar, with Mr. Biden ahead by a single point among those 18 to 29. Either figure is a big shift from Mr. Biden’s 21-point lead in our final poll before the midterms or his 10-point lead in our last national poll in July.And there’s a plausible explanation for the shift in recent months: Israel.As my colleagues Jonathan Weisman, Ruth Igielnik and Alyce McFadden report, young voters in the survey took an extraordinarily negative view of Israel’s recent conduct: They overwhelming say Israel isn’t doing enough to prevent civilian casualties in Gaza, believe Israel isn’t interested in peace, and think Israel should stop its military campaign, even if it means Hamas isn’t eliminated.You might think that the young voters with these progressive or even left-wing views would be among the most likely to stick with Mr. Biden. At least for now, that’s not the case. The young Biden ’20 voters with anti-Israel views are the likeliest to report switching to Mr. Trump.Overall, Mr. Trump is winning 21 percent of young Biden ’20 voters who sympathize more with Palestinians than Israel, while winning 12 percent of other young Biden ’20 voters. In an even more striking sign of defections among his own supporters, Mr. Biden holds just a 64-24 lead among the young Biden ’20 voters who say Israel is intentionally killing civilians, compared with an 84-8 lead among the Biden ’20 voters who don’t think Israel is intentionally killing civilians.It’s possible that the kinds of young voters opposed to Israel already opposed Mr. Biden back before the war. That can’t be ruled out. But it’s still evidence that opposition to the war itself is probably contributing to Mr. Biden’s unusual weakness among young voters.Here are a few other findings from the poll:Biden ahead among likely voters?Even though he trails among registered voters, Mr. Biden actually leads Mr. Trump in our first measure of the 2024 likely electorate, 47 percent to 45 percent.If you’re a close reader of this newsletter, this might not come completely out of nowhere. Our polls have consistently shown Mr. Biden doing better among highly regular and engaged voters — especially those who voted in the last midterm election. In those polls, the most heavily Republican voters have been those who voted in 2020, but not 2022. It helps explain why Democrats keep doing so well in low-turnout special elections even though they struggle in polls of registered voters or adults.But in this particular poll, the split isn’t just between midterm and non-midterm voters. It’s between people who voted in the 2020 general election and those who didn’t. Mr. Biden leads by six points among voters who participated in the 2020 election, while Mr. Trump holds an overwhelming 22-point lead among those who did not vote in 2020. In our estimation, needless to say, 2020 nonvoters are less likely to vote in 2024, and that’s why we show Mr. Biden ahead among likely voters.It’s an intriguing pattern, but there’s good reason for caution here.For one: Our previous polling hasn’t shown anything this extreme, including our battleground polling conducted eight weeks ago. That doesn’t mean it’s wrong, but our sample of 2020 nonvoters includes only 296 respondents — a sample that’s too small for any serious conclusions.For another: The people who voted in 2020 reported backing Mr. Biden over Mr. Trump by 10 points in the 2020 election, 51 percent to 41 percent. In reality, Mr. Biden won by 4.5 points.Now, there’s a good reason respondents might have been less likely to report backing Mr. Trump in our poll: We concluded the substantive portion of the survey with a series of questions about Mr. Trump’s coming legal battles, including whether he committed crimes, whether he’ll be convicted, whether he should go to jail and so on. Then, at the very end of the survey, we asked them how they voted in 2020.It’s possible these questions about Mr. Trump’s legal problems made his supporters less likely to admit supporting him in the 2020 election. Indeed, registered Republicans with a record of voting in 2020 were three times as likely as Democrats to refuse to tell us whom they supported in the last presidential election. But it’s also possible that our sample really does just contain too many Biden ’20 voters with respect to nonvoters, yielding a lopsided shift in his direction among likely voters.The underlying data still looks mostly normal.Every time I see what looks like a crazy result — such as Mr. Trump leading among young voters or a nearly 30-point gap between 2020 voters and nonvoters — I think that I’m going to peer deeper into the data and see the signs that something is off.I haven’t seen it yet.In fact, this survey has a more Democratic sample of young people by party registration than in the past, but a much more Trump-friendly result.A similar story holds for the 2020 nonvoters. They may back Mr. Trump by a wide margin, but 27 percent are registered as Democrats compared with 17 percent as Republicans. Mr. Trump nonetheless leads among them because Mr. Biden has only a 49-34 lead among registered Democrats who didn’t turn out in the 2020 election. He has an 83-8 lead among registered Democrats who did vote.A mere 49-34 lead for Mr. Biden among Democratic nonvoters sounds pretty far-fetched, but it’s at least easy to imagine why these kinds of Democrats might be less likely to support Mr. Biden. If you’re a Democrat who didn’t vote in 2020, you probably aren’t as vigorously and passionately opposed to Mr. Trump as those who did show up. Nonvoters also tend to be young, nonwhite, less educated and have low incomes — all groups Mr. Biden has struggled with. They also tend to be less partisan and less ideological, and therefore may be less loyal to the party.But for now, it’s just one relatively small data point. And curiously, it’s a data point we might never get a chance to validate. Nonvoters don’t vote, after all. In all likelihood, people with a robust track record of voting will play an outsize role in the election, and at least in this poll, that’s good news for Mr. Biden. More