There is nothing incompatible between differences in state election laws and the concept of a national popular vote for President. That was certainly the mainstream view when the U.S. House of Representatives passed a constitutional amendment in 1969 for a national popular vote by a 338–70 margin. That amendment retained state control over elections.
The 1969 amendment was endorsed by Richard Nixon, Gerald Ford, Jimmy Carter, and various members of Congress who later ran for Vice President and President such as then-Congressman George H.W. Bush, then-Senator Bob Dole, and then-Senator Walter Mondale.
The American Bar Association also endorsed the proposed 1969 amendment.
The proposed 1969 constitutional amendment provided that the popular-vote count from each state would be added up to obtain the nationwide total for each candidate. The National Popular Vote compact does the same.
Under the current system, the electoral votes from all 50 states are co-mingled and simply added together, irrespective of the fact that the electoral-vote outcome from each state was affected by differences in state policies, including voter registration, ex-felon voting, hours of voting, amount and nature of advance voting, and voter identification requirements.
Federal law requires that each state certify its popular vote count to the federal government (section 6 of Title 3 of the United States Code).
Under both the current system and the National Popular Vote compact, all of the people of the United States are impacted by the different election policies of the states. Everyone in the United States is affected by the division of electoral votes generated by each state. The procedures governing presidential elections in a closely divided battleground state (e.g., Florida and Ohio) can affect, and indeed have affected, the ultimate outcome of national elections.
For example, the 2000 Certificate of Ascertainment (required by federal law) from the state of Florida reported 2,912,790 popular votes for George W. Bush and 2,912,253 popular vote for Al Gore, and also reported 25 electoral votes for George W. Bush and 0 electoral votes for Al Gore. That 25–0 division of the electoral votes from Florida determined the outcome of the national election just as a particular division of the popular vote from a particular state might decisively affect the national outcome in some future election under the National Popular Vote compact.
The 1969 constitutional amendment, endorsed by Richard Nixon, Gerald Ford, Jimmy Carter, and members of Congress who later ran for Vice President and President such as then-Congressman George H.W. Bush, then-Senator Bob Dole, and then-Senator Walter Mondale, and The American Bar Association and, more importantly, the current system also accepts the differences among states.