> I've never really understood liberal opposition to the possession of firearms in the United States, given the country's founding principles.
You are making an unjustified leap in assuming that "liberals" are in opposition to owning firearms.
Most liberals have no real problem with people owning guns. You don't hear the vast majority of people clamoring to ban hunting rifles or shotguns.
What most people (liberal and conservative) want are sensible regulations to prevent the genuinely mentally ill or disturbed from getting guns and ammo.
The problem is that the NRA plays outrage politics in order generate funding for itself and this short circuits any useful discussion. Although, it is looking like the NRA has just been functioning as a Russian stooge for quite a while now.
As for opposing a coup? That's gonna be tough. The US is BIG. No amount of firearms in Arizona is going to help opposition in Washington DC to fight off a coup.