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.

Questions about France's diversity have played a central role in the campaign for two-round presidential elections April 22 and May 6, particularly about integrating France's millions of Muslims, most of whom have family roots in former French colonies in Africa.

Polling institutes like IFOP, which has been running a much-watched daily survey of voting intentions, are full of data on how the candidates' support breaks down by age, sex, political affiliation and income level _ but not by ethnic group.

"We need to drop this French hypocrisy," said Kamel Hamza, the head of an association of elected officials from ethnic minority backgrounds. "We've been talking about diversity for 30 years, it's time to finish this debate. If the U.S. and others can do it, why not us?"

The French sensitivity over racial and religious polling stems in part from the French constitution, which declares all French citizens equal "without distinction of origin, race or religion." The history of the Holocaust and the deportation of 76,000 French and other European Jews to Germany by France's collaborationist Vichy regime during World War II also contributes to the French hesitancy to count its citizens by race or religion.

Unlike the more multicultural models in Britain or the U.S., France maintains that it's easier to integrate newcomers if every citizen is considered just French, and not classified in an ethnic community.

But France's method has not spared the country racial tensions. And researchers say they are handicapped by this inability to make head counts based on religious or ethnic factors and have pressed for permission to do so.

Because of the law, there are no definitive figures in France on how many people of Arab or Asian descent live here, or how many Catholics or Muslims.

Francois Heran, one of France's top demographers, says the law does allow for certain exceptions that pollsters can take advantage of, but those who do are "very rare." He says the law's complexity and the controversy around ethnic profiling dissuade most people from this type of research.

Among the few research teams to overcome the general taboo around classifying people by ethnicity are Patrick Simon and Vincent Tiberj, who released a recent study based on surveys of 22,000 minorities around France. The study finds that most non-white French citizens are likely to vote left, but doesn't mention any specific candidates.

The surveys were conducted in 2008-2009 on a voluntary, anonymous basis, which means that some of its extrapolations are based on "guesses" about the population at large, Simon acknowledged in an interview Monday. But it's still one of the most extensive studies ever in France on minority populations.

Simon said he hopes "there's a revolution at hand" in terms of allowing study of ethnic groups. "We must have a national reflection on discrimination, how we define minorities, to understand our society."

President Nicolas Sarkozy, whose father was a Hungarian immigrant, once considered then later rejected affirmative action for France. In a significant departure from French practice, he raised the possibility that scientists might begin gathering statistics on ethnicity.

A report ordered up by the government's commissioner for diversity and equality supported such a change to allow scientists more leeway, but after drawing criticism has never been implemented.

Hamza, whose association has campaigned for more flexibility, expressed frustration that the report "has stayed in a drawer" since it was published in February 2010.

Long ignored, France's diversity topped the political agenda after 2005 riots in poor housing projects exposed deep anger among people of immigrant origin and revealed the extent of discrimination in France.

The election of Barack Obama as America's first black president sparked renewed soul-searching about why so few ethnic minorities rise to the top in France.

Socialist presidential candidate _ and poll favorite _ Francois Hollande is so opposed to counting people by race that he even proposed deleting the word "race" from the French constitution. The word is mentioned once in the constitution, in the passage declaring all citizens equal regardless of race, and in subsequent laws forbidding racist acts.

Hollande, in a rally last month with voters from France's overseas territories _ where populations are largely non-white _ argued that the word itself has a fraught meaning. "We know only one single race, one single family: the human family," he said.

The taboo on making ethnic distinctions reflects a "fundamental difference in political systems" between France and the United States, said Nicholas Dungan, a senior fellow at Washington D.C.-based think tank Atlantic Council.

"It's not the way French democracy works, to break it down by race," Dungan said. "You don't get the same degree of slicing and dicing as you do in the U.S."

The restrictions placed on pollsters change the way French presidential campaigns are run, Dungan said. "They are engineered to appeal to people on issues that affect a broader group, rather than smaller subgroups."

Whereas in the U.S. primary campaigns appeal to the party faithful before broadening out in the general election, "French campaigns start in the center and stay there," Dungan said. "They focus less on marginal voters."

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Angela Charlton in Paris contributed to this report.