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Bernie Sanders’s support in Iowa may not be as strong as it seems.

That’s the gist of Nate Cohn’s dive into the data, which shows that Sanders’s lead in several polls—touted by the candidate himself—can be chalked up to random-digit-dialing methodologies that corral voters who are not proven caucus-goers or may not even be registered to vote. In fact, when surveys are conducted of registered voters, those who “definitely” plan to caucus, or those who have caucused before, Hillary Clinton consistently comes out on top.

Of course, Sanders’s lead in the polls could translate into reality if his campaign can get new, younger voters to caucus. But as Cohn points out, Barack Obama, who brought in a lot of new voters himself, was leading among those “definitely” Iowa voters at this stage of the race in 2008, according to The Des Moines Register.

Further complicating Sanders’s chances is the very complex nature of caucus system. As Alex Seitz-Wald points out, Sanders’s supporters are geographically located in such a way as to make their support less consequential. This is where the much-discussed ground game comes in: the ability to accrue delegates from less populated areas across the state, not just urban centers and student hubs. And Clinton would appear to have the advantage there, too.