Lawsuit as soon as they attempt to use to apportion representation. Any citizen in U.S. has standing.
(a) Illegals have always been counted for purposes of apportionment.
(b) The citizenship question has not been on the regular (short-form) census since 1950.
(c) Even under Trump's now-abandoned plan, illegals who were counted in the census were going to be counted for purposes of apportionment; it was just hoped that some illegals wouldn't fill out the form.
(d) The Constitution says that apportionment must be based on "counting the whole number of persons in each State."