Well organized, determined Romans routinely crushed enemies who outnumbered them. Your imaginary 100 million Americans are not organized and the number is highly inflated.
Napoleon used the formula that 1/3 were with him, 1/3 against him, and 1/3 simply didn't care. So yes that gives you a 100 million. But, there is that pesky 100 million who are against you. That and a professional army is a big obstacle.
Maybe in Massachusetts, but come down here to the South & Southwest and I imagine you’ll think differently, especially Texas. The Romans were very well drilled and trained, and yes, we have people like that in our present day military, but I don’t see very many of them fighting their neighbors. The Roman legions mostly fought outlanders or slave rebellions, towards whom they had little or no empathy. And if it is so easy to quash a revolt or rebellion, please explain Iraq, the IRA, Afghanistan, Vietnam, the Moros, Chechnia, etc., etc...
Insurgent warfare hasn’t worked out real well for the U.S. military over the last 50 years.
Look what a few Taliban are doing to humble the modern American and NATO forces. 100 million of us are disorganized now, but they know their native terrain better than the military and will lie in ambush to easily put them down. Caesar had to import Germans to fight against Romans, because Roman Soldiers would not do that. The American Soldier will frag the officers who order them to kill Americans.
Those pesky 100 million are supposedly not gun-owners. What are they going to do, build spears, bows and arrows and zip guns?