The decision to add a citizenship question is obviously about political power. Trump is counting on noncitizens to refuse to answer the census, thus affecting apportionment of congressional seats to states and block grant formulas of federal money to states and localites
But, the Census Bureau is obligated to enumerate the entire population, so the first consequence will be that the Census Bureau will have to deploy more field interviewers to affected communities, thus increasing the cost of the Census
Likely the field interviewers will meet resistance in some communities, and Census has limited funds to do follow up interviews. So while the cost run ups will meet some ceiling of available funds, thereby undercounting noncitizen communities
While Republicans may seek to reap political benefits from an undercount of noncitizen populations, this is a short term political gain for long term public policy harm
Some non-political uses for census data that are put at risk:
-identifying disease outbreaks
-knowing where best to allocate police force
-knowing where to locate a new business in a growing population area
and so on...
-identifying disease outbreaks
-knowing where best to allocate police force
-knowing where to locate a new business in a growing population area
and so on...
E.g., because the denominator of the disease rate is incorrect, government might rush aid to an affected noncitizen community that has an artificially high rate due to fewer census responses, thereby wasting resources while the real outbreak in a citizen community is ignored
Corporations and government will underestimate population growth in noncitizen communties. Fewer private and public services will be deployed, leading to fewer opportunities (creating an environment for more crime), less education (creating more poverty for citizen children), etc
Hopefully groups can educate communities how the Census Bureau operates in strict confidentiality and will not allow citizenship status to be released outside the Bureau. Yes, a law could be passed to change that, but would require 3/5th majority to overcome the Senate filibuster
The Census Bureau operates carefully when it comes to changing the decennial census or any of the numerous surveys. They field test changes to estimate effects. Sec. Ross made a decision without any such testing. Claiming "no evidence" there will be ill effects is like ¯\_(ツ)_/¯ 
People who do surveys for a living sweat seemingly trivial details that affect the quality of survey responses. The decennial census, even though a full enumeration, shares several traits with survey questionnaires
