The census counts those living in the US regardless of their immigration status. That census count is used to determine Congressional representation. Plus, anyo

The census counts those living in the US regardless of their immigration status. That census count is used to determine Congressional representation. Plus, anyone born in the US, regardless of immigration status of their parents, is a legal citizen and in eighteen years, a voter. Politicians might not be able to see past the next election cycle, but political parties play the long game (e.g. "the Southern Strategy")