16% of Americans live in rural areas.
With National Popular Vote, big cities would not get all of candidates’ attention, much less control the outcome.
The population of the top five cities (New York, Los Angeles, Chicago, Houston and Philadelphia) is only 6% of the population of the United States and the population of the top 50 cities (going as far down as Arlington, TX) is only 15% of the population of the United States.
Suburbs and exurbs often vote Republican.
If big cities controlled the outcome of elections, the governors and U.S. Senators would be Democratic in virtually every state with a significant city.
A nationwide presidential campaign, with every vote equal, 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 vote is equal, a campaign must be run everywhere.
With National Popular Vote, when every vote 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 Wyoming, or for a Republican to try it in Wyoming or Vermont.
Even in California state-wide elections, candidates for governor or U.S. Senate don't campaign just in Los Angeles and San Francisco, and those places don't control the outcome (otherwise California wouldn't have recently had Republican governors Reagan, Dukemejian, Wilson, and Schwarzenegger). A vote in rural Alpine county is just an important as a vote in Los Angeles. If Los Angeles cannot control statewide elections in California, it can hardly control a nationwide election.
In fact, Los Angeles, San Francisco, San Jose, and Oakland together cannot control a statewide election in California.
Similarly, Republicans dominate Texas politics without carrying big cities such as Dallas and Houston.
There are numerous other examples of Republicans who won races for governor and U.S. Senator in other states that have big cities (e.g., New York, Illinois, Michigan, Pennsylvania, and Massachusetts) without ever carrying the big cities of their respective states.
With a national popular vote, every vote everywhere will be equally important politically. There will be nothing special about a vote cast in a big city or big state. When every vote is equal, candidates of both parties will seek out voters in small, medium, and large towns throughout the states in order to win. A vote cast in a big city or state will be equal to a vote cast in a small state, town, or rural area.
Candidates would need to build a winning coalition across demographics. Any candidate who ignored, for example, the 16% of Americans who live in rural areas in favor of a “big city” approach would not likely win the national popular vote. Candidates would have to appeal to a broad range of demographics, and perhaps even more so, because the election wouldn’t be capable of coming down to just one demographic, such as waitress mom voters in Ohio.
With National Popular Vote, every vote, everywhere, would be politically relevant and equal in presidential elections. Wining states would not be the goal. Candidates would need to care about voters across the nation, not just undecided voters in the current handful of swing states.
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.