PARIS—Ahead of France's presidential elections, you might find polls showing how factory workers or university students are likely to vote—but you'd be hard-pressed to find data on which candidates black or Muslim voters prefer.

A 1978 law forbids anyone, from government census-takers to private researchers, from counting citizens based on their ethnic, racial or religious background. Though the law allows a few exceptions, it makes it virtually impossible for pollsters or political parties to measure nationwide support for candidates among those various constituencies, a situation that would be unimaginable in a U.S. presidential election.     

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