The Supreme Court Tuesday will hear oral arguments in the Trump administration’s appeal of lower-court orders forbidding it to ask a citizenship question in the 2020 census. The justices’ task in Department of Commerce v. New York won’t be difficult: The law and facts overwhelmingly support the administration. But the case is a proxy for future battles over redistricting and reapportionment, vital components of American democracy that determine the balance of political power within and among states.

The Census Act grants...