(For clarity: Applicant - the adult financially responsible for the lease. Occupant - anyone over the age of 18 that will be residing with the Applicant but is not financially responsible for the lease.)
Is it legal for a landlord to refuse a lease application for the sole reason that an Occupant included in the application is an undocumented immigrant despite the Applicant being a US Citizen?
Examples would include a US Citizen who is seeking housing to live with his/her legal spouse or adult child who is an undocumented immigrant. In this case, the Citizen would be the Applicant and the spouse or child would only be included in the rental application as an Occupant.
One justification some landlords give is that they are unable to process a background or credit check without an SSN, however some resources such as this one seem to make that defense untenable.
I'm not an attorney but it seems to me that refusing to lease housing to an undocumented immigrant, who meets all other requirements and is only an Occupant, based solely on their immigration status is nearly equivalent to refusing to sell other necessities such as food, water or clothing to a person simply because of their immigration status. Or perhaps even worse, refusing to sell those necessities to a Citizen who is buying those necessities with or for an undocumented immigrant.
(Potentially relevant is the case of Lozano vs City of Hazleton where the Supreme Court denied to City of Hazleton their appeal to an earlier decision that ruled as unconstitutional a city ordinance that prohibited landlords to rent to undocumented immigrants. The basis for the decision was mostly due to local government being deemed to have overreached into the arena of immigration enforcement - an area explicitly reserved for the Federal government.)