BOSTON—Saturday morning found me at a campus leaders’ conference at Harvard with a crowd of 120 students, the leaders of College Democrats, College Republicans, and campus newspapers from state universities in 43 states. Florida Gators were mixed in with Iowa Hawkeyes, Kansas Jayhawks, Montana Grizzlies, and USC Trojans.

During the first session, I sat bolt upright when pollster John Della Volpe asked the audience how many had land lines. As I looked around the room, just three students raised their hands. I realized that could be the election right there.

Why? Pollsters can’t legally make unsolicited cell-phone calls, so people without land-line phones are much harder to include in surveys. Increasingly, that means it’s hard to reach young voters—government surveys last year found that one in three Americans under 30 doesn’t have a land line, and as Della Volpe’s informal survey showed, that number rises the younger you get. Pollsters are trying to find ways around that hole in their data; for instance, Della Volpe has partnered with Harris Interactive to provide massive online panels for his surveys and to help reach younger voters. Other pollsters are asking for the youngest voter in a household in their calls. By and large, though, the surveys are far from perfect.

The unscientific survey of the Harvard audience underscored just how big a challenge pollsters face this fall. All of the polls showing the election to be a dead heat could be off by several points if they’re undercounting young voters with cell phones.

This land-line problem was one of the reasons pollsters missed Barack Obama’s big victory in the Iowa caucuses earlier this year, when young voter turnout was four times the level of 2004.

Young voters in this election will likely represent somewhere in the neighborhood of one out of five voters—a substantial chunk by any measure. If the turnout numbers hold true through the fall (the Iowa numbers weren’t an aberration­—in Missouri and Tennessee, youth voting tripled or quadrupled from 2004), then John McCain has a tough road ahead.

The 85 million or so “millennials”—those Americans born between 1977 and 1997—are the largest generation in American history, and they’re just beginning to flex their muscle at the voting booth. In 2006, they proved decisive in the Democratic victories of Jim Webb in Virginia and Jon Tester in Montana. It’s a diverse and socially tolerant generation—one out of three millennials is nonwhite, and their views on stem-cell research and gay marriage are nearly the inverse of their conservative “greatest generation” grandparents.

The latest presidential-preference poll of young voters, conducted by Harvard’s Della Volpe in August before the conventions, found Obama leading among young voters by some 23 points, 55 to 23, with 13 percent undecided. Those numbers are almost unchanged from a poll released in April.

In 2004, young voters were the only age group that John Kerry won, and he won by just nine points.

Sarah Palin might have changed that gap—we’ll have a better sense during the last of the young-voter polls in October—but with that preconvention gap, all of the undecideds would have to break for McCain in order for him to close the gap to John Kerry’s 2004 lead. Unless he’s able to do that, he’ll lose the election.

More interesting in Della Volpe’s August poll, though, was that young voters trusted Obama over McCain on nine out of ten issue areas—from terrorism to Iraq to energy to immigration. The only area where McCain eclipsed Obama was the question about whom respondents trusted more to handle being commander-in-chief. On the question of bringing change to Washington—the fundamental issue that this campaigns seems to be boiling down to—43 percent chose Obama and just 13 percent chose McCain.

As he goes into the first debate this week, McCain must work to reassure young voters that he stands for change and that he can lead their generation—one that’s less white and more socially tolerant than any that came before it. Here’s a free tip for the McCain campaign in reaching millennials: Don’t try to call them on their land-line phones.

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