There's a story I read, though I can't remember it in exact detail, but the basic gist of it was this.
I believe it took place in California during a drought, or it may have even been a hypothetical as example. One of the big cities, like LA or San Diego, voted to redirect the water being used for irrigation and such by all the farms in proximity. This meant the farms couldn't produce their crops, the farmers couldn't make any money to feed their families, and other places that relied on those crops to feed their populations were also adversely affected.
All because a single area with a high, dense population voted to take what they wanted from a much lower population spread out over a wider area.
It's exactly like the kind of thing depicted in The Hunger Games, as I mention in the meme I posted above in reply to another thread.
Plus, the right to an equal say in the running of the country is the primary reason most states signed on to be members of the country in the first place. The abrogation of that right is one of the main reasons southern states chose to secede from the union, leading to the Civil War.
Get rid of the electoral college, and it wouldn't surprise me if a lot of states attempted to secede NOW.