The millions of suburban Republican voters in California and New York would suddenly become relevant again, thus prompting the Republicans to back-off some of their recent forays into protectionism.
Meanwhile, Democrats would suddenly have to play much harder to Black voters in the South as a key component of winning nationally.
Basically this. The GOP would also initially want to go for safe states that have low voter turnout. There are a lot of raw votes to gain in Oklahoma, for instance, that nobody tries for in the EC system because they don't matter.
