The Trump administration is seeking to add the question and on Tuesday, the high court's conservative majority signaled that they are leaning in that direction.
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The ruling is not just about counting people, it's also about how federal dollars are allocated, among other things.
Illinois Attorney General Kwame Raoul joined at least 19 other state attorneys general in the fight to keep the citizenship question out of the census.
"Population, accurate count of population impacts how some of these infrastructure dollars, Medicaid dollars, dollars for education. You name it, probably in excess of $800 billion of dollars of appropriations," Raoul said.
With the population exodus from Illinois, there are concerns the state could lose not one but two congressional seats if people living here are undercounted.
State Sen. Elgie Sims Jr. (D-Chicago) said there are several bills in Springfield that would allocate $25 or $33 million to help ensure the count is accurate.
"Traditionally, you find that minority communities and poor communities have been hard to count and hard to count populations is really where we need to be spending resources to get an accurate count of those communities," Sims said.
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The outcome of the Supreme Court hearings holds particular concerns for Hispanics.
"There's a real mistrust as to how that information may be used to harm people, to deport people, to have more raids and it just really creates fear in terms of people living and working in the United States," said Sylvia Puente, executive director of the Latino Policy Forum.
The constitution requires an accurate count of everyone living in the U.S., and there are additional concerns this is a partisan effort by the Trump administration to hurt Democratic strongholds.
"We know that if this happens it will heavily undercount, heavy Democratic parts of the United States, it will undercount our large urban centers, and some of our heavily democratic states," Puente said.
The last time a question about citizenship was asked was 70 years ago. The 2020 census will be the first time the census will be conducted only online.