A Republican senator’s proposal to count only United States citizens when reapportioning Congress would cost California five seats and New York and Illinois one each, according to an independent analysis of census data released Tuesday. Texas, which is projected to gain three seats after the 2010 census, would get only one.

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The proposed change would spare Iowa, Louisiana, Michigan and Pennsylvania the expected loss of one seat each. Indiana, Montana, North Carolina, Oregon and South Carolina would each gain a seat.

If every resident — citizens and noncitizens alike — is counted in 2010, as the Census Bureau usually...