Because the presidential selection system currently consists of independent simultaneous statewide votes, state politicians in a single state logically conclude their actions will have little effect on the presidential election’s outcome in other states.
Republican legislators in North Carolina, Michigan, and Wisconsin can act against the apparent desire of the majority of voters in last month’s election without worrying about the effect on their party’s ability to win a national popular vote — because no such vote ever matters.
But things would be different if some states awarded some electors to the national vote winner. Then state officials in both parties (for the first time in American history) would have a ballot-based reason to be concerned about the reaction to their conduct from fellow Americans across the whole country. Anti-democratic moves by either party in any state might shift public opinion against that party in other states. Notorious bad actions by either party even in a single state then might cost the party a national popular vote majority and as a result lose the presidency.
The anachronistic notion that what happens in a single state stays in that state would be eradicated. If the national popular vote mattered then the actions of officials in a single state might be subjected to meaningful judgment in the court of national public opinion.