<p>US CODE Title 10.6958(b):
"Each candidate for admission nominated under clauses (3) through (9) of section 6954 (a) of this title must be domiciled in the State, or in the congressional district, from which he is nominated, or in the District of Columbia, Puerto Rico, American Samoa, Guam, or the Virgin Islands, if nominated from one of those places." </p>
<p>Very straight forward. Congressmen swapping appointments has not happened in at least 30 years or so. It did, in fact, used to be common but no more. They also traded appointments between the academies, a WP for USNA, etc. Academy Admissions Office guards against candidates being appointed from the wrong district.</p>