"The Department of Commerce is not able to determine
definitively how inclusion of a citizenship question on the decennial census
will impact responsiveness. However, even if there is some impact on responses,
the value of more complete and accurate data derived from surveying the entire
population outweighs such concerns. Completing and returning decennial census
questionnaires is required by Federal law, those responses are protected by
law, and inclusion of a citizenship question on the 2020 decennial census will
provide more complete information for those who respond."
The census
has been conducted every decade since 1790 to get a
national head count used most critically to decide the distribution of
congressional representation. At first it was conducted by U.S. marshals, but
later surveys were sent to most American households, with census workers
helping those who didn't promptly return their surveys.
The last time a census form
sent to most American households asked a question about U.S. citizenship was in
1950. That form asked where
each person was born and in a follow-up question asked, "If foreign born —
Is he naturalized?"
In 1960, there was no such question about citizenship, only about
place of birth.
In 1970, the Census
Bureau began sending around two questionnaires: a short-form questionnaire to
gather basic population information and a long form that asked detailed
questions about everything from household income to plumbing. The short form
went to most households in America. The long form was sent to a much smaller
sample of households, 1 in 6. Most people didn't get it.
Starting in 1970, questions about citizenship were included in
the long-form questionnaire but not the short form.
The 2000 census short form asked about race but not citizenship,
which the long form did those who received the long form were asked, "Is this person a
CITIZEN of the United States?" Bill Clinton was President and there wasn’t
a HUE and Cry then.

The 2000 long-form survey, sent to a subset of Americans, asked about citizenship. The more widely distributed census short form that year did not.
The short form kept it simple: name, relationship,
age, sex, Hispanic origin, race, marital status and whether the home is owned
or rented.
The 2000 census short
form asked about race but not citizenship, which the long form that year did
ask about.
In 2005, the census added a new survey, the American Community Survey, conducted every year and
sent to 3.5 million households. It asks many of the same questions as the
census long-form surveys from 1970 to 2000, including the citizenship question.
In 2010 the citizenship
question was removed. The Census it had been replaced by the annual American
Community Survey. The decennial census form asked just 10 questions.
The state of
California has already sued to block the addition of a citizenship
question to the 2020 census. The concern expressed by states with large
undocumented immigrant populations is that asking about citizenship will scare
people off, forms won't get filled out and the count won't be accurate,
affecting federal funding and the number of congressional seats. (Though the
Census Bureau is legally required to keep answers confidential, even
from the FBI and other government entities.) Info Source