>>43972California would not decide the presidency. The popular vote would. There are still tons of Republicans in California, it's just that there's a built-in edge for Democrats there. And if EVERY VOTE MATTERED, then that sort of thing where your state is "safe" blue/red is suddenly no longer a barrier to vote. A red vote in California would matter, it would be counted.
Right now you get random, arbitrary swing states deciding the presidency. It wasn't a singular country when it was formed, but it is now. People see themselves as American, not Nebraskan. Federal powers and authority over the states has been expanded for hundreds of years. It's a singular country.