Equal representation of the states in the U.S. Senate is explicitly established in the U.S. Constitution. This feature cannot be changed by state law or an interstate compact. 

In fact, equal representation of the states in the U.S. Senate may not even be amended by an ordinary federal constitutional amendment. Article V of the U.S. Constitution provides: 
“No State, without its Consent, shall be deprived of its equal Suffrage in the Senate.” 

Thus, this feature of the U.S. Constitution may only be changed by a constitutional amendment approved by unanimous consent of all 50 states. 

In contrast, the U.S. Constitution explicitly assigns the power of selecting the manner of appointing presidential electors to the states. The enactment by a state legislature of the National Popular Vote bill is an exercise of a legislature’s existing powers under the U.S. Constitution.