In recent years, the rise of the National Front party has been a major feature of the French political landscape. Understanding the drivers behind the electoral successes of that party is now crucial for mainstream parties if they want to counter it efficiently. This is no small task, as the vote for the National Front is difficult to estimate for pollsters. Our contribution shows how Open Data can help shed light on the question, by combining voting data with freely available administrative data sets. In particular, we address the important question of the link between turn out and the National Front vote. We show that simple correlations between the two are biased and we estimate the true link by econometric modelling, dealing with endogeneity.