Suppose, for example, that the Utah state legislature decides Salt Lake City shouldn't get US House representation, so they draw the congressional districts such that no district's boundaries include any portion of Salt Lake City.
Would this be legal?
The 14th Amendment says
All persons born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States and of the State wherein they reside. No State shall make or enforce any law which shall abridge the privileges or immunities of citizens of the United States; nor shall any State deprive any person of life, liberty, or property, without due process of law; nor deny to any person within its jurisdiction the equal protection of the laws.
However, there are many citizens who are or were legally prohibited from voting, including minors and felons (still), women (until the 19th Amendment), and blacks (sporadically, until the 15th Amendment outlawed that). Reynolds v. Sims, 377 U.S. 533 and related cases held that districts within a state have to be comparable in population, but does not directly rule on whether a state can decide to leave a certain geographical area entirely out of the subdivision into districts (the issue there was districts that were lopsided, population-wise). §2 of the 14th Amendment does maintain that
when the right to vote...is denied to any of the male inhabitants ... or in any way abridged, except for participation in rebellion, or other crime, the basis of representation therein shall be reduced...
so the state-wide basis for counting representatives would be reduced.
No existing constitutional amendment, federal law, or supreme court ruling expressly denies this possibility. The reasoning of Reynold v. Smith does, however, strongly point to the outcome that SCOTUS would find such an exclusion to be unconstitutional, because as that ruling says at the top of §II,
Undeniably, the Constitution of the United States protects the right of all qualified citizens to vote, in state as well as in federal, elections.
Being a minor or a felon are the only two recognized grounds for depriving a citizen of the right to vote in Congressional elections in their state.
Reynolds v Sims 377 U.S. 533 (1964) decided that the equal protection clause required “one person, one vote”. In practice this means that electoral districts must have (approximately) the same number of voters.
While it didn’t address disenfranchisement directly, its difficult to see how excluding otherwise eligible voters meets the “one person, one vote” test the case laid down.
