(CNN) -- The Census Bureau expects that 630,000 households may not complete the 2020 Census because of a potential question asking about respondents' citizenship question, it told the White House recently.
The estimate includes both those who return the survey but leave the citizenship question unanswered, and those that do not reply at all.
In both cases, Census Bureau employees follow up with mailings and in-person visits -- the most expensive part of the decennial population count.
The document is a procedural filing with the White House Office of Management and Budget justifying the information the Census Bureau plans to collect. It notes that the citizenship question will be included "should the government prevail in pending litigation."
The Supreme Court is set to hear arguments in a challenge to the question this spring.
The document makes a robust case for the citizenship question, noting that the 630,000 households represent "0.5 percent of the entire estimated population" which and that the follow-up costs "would be easily accommodated" in its budget.
But when Census Bureau leadership first considered the 630,000 figure, they worried it would make the data less reliable and "could increase the cost of the 2020 Census by at least $27.5 million."
That was the argument career Census leadership made in a January 2018 memo to Commerce Secretary Wilbur Ross recommending that a citizenship question not appear on the 2020 survey.
They called the $27.5 million "a conservative estimate" because it figures in only households with non-citizens. If households of entirely American citizens also choose not to respond, the workload and costs will increase, it says.
The new filing, dated February 11 and flagged on Tuesday by an NPR reporter, says the Census Bureau now believes the question will not impact the quality of the data.
"To clarify, the Census Bureau has identified no credible quantitative evidence that the addition of a citizenship question would impact the net undercount of the 2020 Census," the document reads.
It notes Ross "consulted extensively with the Census Bureau and a broad array of stakeholders" including "more than 24 diverse, well-informed, and interested parties representing a broad range of views."
99.1 percent of the more than 137,000 formal public comments on whether to include the question were in opposition.
Ross explained the question as a request from the Justice Department needed to enforce voting protections.
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