“Candidates would need to care about voters across the nation, not just undecided voters in the current handful of swing states.”
You keep claiming this as though it’s fact, when any farmer, if forced to choose, would harvest where the crop is most dense. Voter wise, that is in the big cities.
A nationwide presidential campaign, with every voter equal, and every vote counting for the candidate for whom it was cast, would be run the way presidential candidates campaign to win the electoral votes of closely divided battleground states, such as Ohio and Florida, under the state-by-state winner-take-all methods. The big cities in those battleground states do not receive all the attention, much less control the outcome. Cleveland and Miami do not receive all the attention or control the outcome in Ohio and Florida.
The itineraries of presidential candidates in battleground states (and their allocation of other campaign resources in battleground states) reflect the political reality that every gubernatorial or senatorial candidate knows. When and where every voter is equal, a campaign must be run everywhere.
With National Popular Vote, when every voter is equal, everywhere, it makes sense for presidential candidates to try and elevate their votes where they are and aren’t so well liked. But, under the state-by-state winner-take-all laws, it makes no sense for a Democrat to try and do that in Vermont or Oklahoma, or for a Republican to try it in Oklahoma or Vermont.
The main media at the moment, TV, costs much more per impression in big cities than in smaller towns and rural area. Candidates get more bang for the buck in smaller towns and rural areas.