Arizona could gain an extra congressional seat if President Donald Trump succeeds in implementing a mid-decade census, excluding illegal immigrants, but the cost, logistics and legality of the whole idea cast doubt it will happen.

The presidential proposal calls for an unprecedented effort to count only some U.S. residents, upending the practice of counting everyone and inviting an unsettling error rate that could yield results that might not deliver as many Republican seats as Trump presumably wants, experts say.

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