If America were to move to a proportional system one problem which would be raised would be the Senate. Article Five of the U.S Constitution states:
The Congress, whenever two thirds of both houses shall deem it necessary, shall propose amendments to this Constitution, or, on the application of the legislatures of two thirds of the several states, shall call a convention for proposing amendments, which, in either case, shall be valid to all intents and purposes, as part of this Constitution, when ratified by the legislatures of three fourths of the several states, or by conventions in three fourths thereof, as the one or the other mode of ratification may be proposed by the Congress; provided that no amendment which may be made prior to the year one thousand eight hundred and eight shall in any manner affect the first and fourth clauses in the ninth section of the first article; and that no state, without its consent, shall be deprived of its equal suffrage in the Senate.
One solution would be to make the Senate largely powerless like the British House of Lords.
Another solution, which might be considered to follow the letter but definitely not the spirit of the law, would be to give every state no representation in the Senate(thus keeping “equal suffrage” for every state!), and making it instead some kind of appointive body, also like the House of Lords. I’m sure Anthony Kennedy could handle the legal reasoning.