It’s common to talk about the turnout challenge facing Donald Trump, who clearly fares well among those who don’t vote regularly. But the candidate with the biggest turnout challenge in this cycle is probably Bernie Sanders.

Mr. Sanders appears to be extraordinarily dependent on turnout from infrequent voters, even more than Democrats have recently been in general elections, and maybe more than Barack Obama in the 2008 Iowa caucuses — mainly because his support is so strong among the young.

As Mr. Obama can attest, you can turn out and win with irregular voters — and Mr. Sanders could prove to have the enthusiasm and organization needed to do the same. In one example, as Jason Horowitz and Yamiche Alcindor reported in The Times, the Sanders campaign has plans to send rental cars, vans and buses to carry students who are from Iowa back to their hometowns for caucus day.

But the scale of Mr. Sanders’s turnout challenge is unusually large. Compared with the supporters of Hillary Clinton, his are far less likely to report that they intend to vote; they have less history of voting; and they come from demographic categories who turn out in low numbers. This all adds considerable uncertainty to the pre-election polls, which always struggle to determine who is or is not likely to vote.