1) the part that specifically says the states shall choose their own presidential electors. The supreme court has said delegating this power to a popular (statewide) vote is permissable, but when delegated, the vote must adhere to 1 person 1 vote principles. A delegated-power that incorporates the votes of other states likely violates this principal, and is therefore unconstitutional.

2) the part that says no interstate compacts without express approval of congress