The Senate made the right decision. The purpose of the census is not to count citizens, but to count and record demographic data of people living in the country. Sure, it sounds like an entirely reasonable question and the data would be good, but it's already hard enough to get data on illegals and the presence of this question would likely freak them out more. It's a lesser of two evils situations: ask for citizenship data and only be able to record half the number of illegals (and, since the govt is expected to go to great lengths to get this data, end up costing a lot more), or don't ask it and get a more accurate census.