This article exploring what campaigns would look like under a national popular vote asks the right questions, but I think requires more knowledge of the cost of advertising and the cost on persuading voters. These are both challenging topics because decent information is not accessible.
But logic tells us that if it’s cheaper for either party to persuade a truly undecided voter in North Dakota than in New York City then North Dakota will get the marginal dollar of spending and New York City will not.
Furthermore, if the Democratic Party can more cheaply collect 100,000 votes in South Dakota than in Los Angeles then it will head for the (Black) Hills.
Similarly, if the Republican Party can pick up an extra million votes in California more readily than getting another million votes in Texas then it will focus for the first time in California.