At least a dozen states plan to sue the Trump administration over its decision to add a citizenship question to the 2020 Census, with the attorneys general of New York and California—populous states with large immigrant populations—leading the charge. But the damage of a potential undercount won’t be confined to the coasts.

Before Commerce Secretary Wilbur Ross announced the citizenship question on Tuesday, the Democratic co-chair of the House Census Caucus had already proposed a bill to block la…