If republicans and democrats are going to sort themselves into their own states, what does this mean for the future of the US federal government? Are we going to end up with the democrat and republican states forming two separate countries?
However, if dissolution did happen, it wouldn't necessarily be a bad thing. Most of America's current political strife and dysfunction is caused by incompatible, and increasingly divergent, visions for America's future. Splitting the country is the only way for those two visions to get what they want without persecuting (or killing) large numbers of people.
Thing like drinking age, homeless accommodation or medical services can be perfectly well driven by states.
A national popular vote would drastically shift the race to the most populous metro markets in the US: NYC (~19M), LA (~13M), Chicago (~9M), Dallas (~8M), Houston (~7M), and DC (~6M). Despite their heavy leanings to Democrats, the raw number of votes in these markets and the cost to reach a voter still makes them competitive areas.
Such a shift in election method would likely move the 'center' of the US presidential political system towards issues that dominate the large metro markets. This further alienates rural voters and less populous states, making Senate policy less representative of the US voter.