I've discussed this before, but I'll elaborate on a few points:
i) It wouldn't surprise me if, at some point, Democrats try to confiscate guns. There's an incremental strategy. You can see Obama attempting to build a case.
If there was an attempt to confiscate guns, that could trigger a civil war. At the very least it would lead to massive civil disobedience.
In addition, I think many liberals would welcome a civil war. With the gov't on their side, they think their side would win. That would be an opportunity to stamp out the "rightwing" once and for all time.
ii) I've seen liberals mock the idea that an armed citizenry is any check on gov't. Surely an armed citizens are no match for the US military.
iii) That, however, overlooks a number of complications. There's the question of which side our soldiers would take in an American civil war.
And I don't just mean soldiers quitting to fight for the rebels. You might have a sizable percentage of soldiers (as well as police, FBI, NSA) who keep their jobs, but assist the rebels from the inside. Spies and sympathizers.
iv) The US military has awesome firepower at its disposal. Remember, though, that we're discussing the scenario of a civil war on American soil. You don't fight that the same way you fought the Japanese in WWII, or the Viet Cong in that war. It's one thing to destroy someone else's country, it's quite another thing to destroy your own.
v) Unlike the American Civil War (1861-1865), which was a regional war with fairly clear geographical boundaries, in the scenario we're considering, the rebels would be distributed nationwide. In the American Civil War, the Union could treat the Confederate states as if they were a foreign country, like a border war. In addition, the Southern economy was largely agrarian whereas the Northern economy was largely industrial. The North was able to damage the South in a way the South was unable to damage the North.
So, for instance, Sherman could get away with burning Atlanta. But if you had a nationwide Civil War, then the gov't can't afford to use the same scorched-earth tactics. It's not going to firebomb cities or suburbs, is it? It's not going to send cruise missiles to take out residential skyscrapers, is it?
With rebels dispersed in urban and suburban population centers all across the nation, military firepower is fairly useless. Tanks, bombs, missiles, &c., are too destructive. The battlefield isn't foreign territory, but your own cities and suburbs.
The rebels would resort to guerilla warfare. Blend into major population centers. To combat that would require US troops going door-to-door to ferret out rebel cells.
vi) The modern-day US economy is far more fragile than it was in the 19C. Just consider how dependent we've become on electronic communications, including mobil networks.
In the past, cities were largely suppled by local farms. Communities were more independent. Now stuff is trucked in from out of state. If interstate commerce began to break down, if the power grid failed, cities would begin to shutdown. Just imagine what would happen nowadays if mobil networks were disrupted.
There's no telling in advance how damaging the effort would be. The degree of popular support. Inside help. Hacktivists.