The US is already 50 countries. The states have an incredible amount of power over their citizens. They’ve always been a confederation. States engage in trade wars with each other. California, for example, has used its massive economy to influence interstate trade and declare de facto federal laws on several occasions. We have expiration dates because of New Jersey. Some counties entire economy relies on sales to people from neighboring states where certain goods are banned. Texas, infamously, has its own electrical grid and throws a tantrum about seceding every few decades. Utah is one pair of magic underwear away from being a theocracy.
The states have factions and inter state pacts. The thing you’re missing is that constantly fighting with each other is what keeps them together. The US was never and will likely never be a homogeneous culture, and the people who founded it knew that, and encoded laws in such a way as to use that conflict as a source of opposing-force balance.