“Candidates would need to care about voters across the nation, not just undecided voters in the current handful of swing states.”
You are talking polling (what voters already think without/before the candidate says anything else) and statistics and “campaigning”, not reality, and certainly not the reality of whether or not voters care more about to whom a candidate is makimg a pitch (they don’t) or what they think of the pitch no matter to whom they are making it (they do).
As I noted in my initial response in this thread; during the last eight presidential elections, the eventual winner obtained a majority of votes in a majority of counties all across the country.
And that is how the winners actually obtain a majority of electoral votes. The amass more majorities of more places in the country, even when the “popular vote” is something less than a majority. This was true when Bill Clinton only got 43% of the vote and it was true in the election of 2000. The winner accumulated the assent from “more of the country”, not just the places with the “most votes”.
Reality is, the winner now does NOT need to accumulate the assent from “more of the country”, not just the places with the “most votes”.
With the current state-by-state winner-take-all system of awarding electoral votes, it could only take winning a bare plurality of popular votes in the 11 most populous states, containing 56% of the population of the United States, for a candidate to win the Presidency — that is, a mere 26% of the nation’s votes.