[LexBaird]

Since these matters are explicitly given over in the Constitution to Congress my bet is no court would declare Congressional actions regarding membership to be "unconstitutional".

But they are not exclusively given over to Congress. The 17th A. gives the power to fill vacancies to the State Executive. If the State brings the case that this constitutes a vacancy, then "[i]n all Cases ... in which a State shall be Party, the supreme Court shall have original Jurisdiction."

[justshutupandtakeit]

That doesn't get around the fact that only the Senate has the power to declare a vacancy.