Via US News:
THE CENSUS BUREAU IS pursuing a legal loophole that its officials believe would allow them to temporarily hire non-U.S. citizens as part of its efforts to reach populations that are difficult to count, including in non-English speaking and immigrant communities.
People employed by the Census Bureau, including those hired temporarily, are considered federal employees and are legally required to be U.S. citizens. Specifically, the annual Appropriations Act prohibits the use of appropriated funds to employ noncitizens within the U.S.
Some exceptions exist, however, that allow the agency to hire translators temporarily, as well as to hire people admitted to the U.S. for permanent residence who are seeking citizenship, to hire people admitted as refugees or granted asylum, and to hire people for up to 60 days on an emergency basis.