Washington (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.

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