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On June 12, Iranian voters will choose among four leading candidates for president, but their real choice is singular: whether to continue on the course plotted by Mahmoud Ahmadinejad. This ought to be a no-brainer: Ahmadinejad has made a mess of the economy, clamped down on political dissent and social freedoms, militarized the state, and earned the enmity of much of the world. In large Iranian cities, even those who voted for him in 2005 are almost unanimous in their disappointment. But in Iranian elections, demographics are everything. It all depends on who, exactly, shows up to the polls.

Iran has its own version of the Red State dynamic. Although just 35 percent of the population lives in rural villages, which are more traditional and conservative than the cities, these...