In fact, the purpose of having a constitutional framework is precisely preventing one ruler to go all power hungry and change everything on his own, that's why most dictatorships come after a war, or in countries with paper-strong institutions that can be blown up just by breathing too hard around them. If you think the US would allow someone to just randomly supress the Senate or outlaw elections out of the blue, you have a ridiculously unrealisticly low faith on them.
And, despite all this, if somehow an evil President took over the US, I'm pretty sure guns would help stop the largest army in the world- all those tanks and drones and missiles and internet control over the water supplies have nothing to do against a good old rifle, right? Right? In fact, they probably would not be necessary if there are massively gigantic demonstrations.
As (I would have thought unnecessary) clarification - my response was to those on this thread who are demanding that the US government be granted the power to ban guns. I didn't even address whether such a thing is possible, since that's a secondary issue - the primary issue is merely that it's desired, and further that that desire is treated as legitimate, and the destruction of rights necessary to enact it justified.
First - the mere fact that people demand that it be done is problematic, not to mention depressing, simply because it's evidence of the complete failure of so many to even begin to grasp the notion of rights. Second - and to your point - such a thing IS possible, and explicitly possible with sufficient public support (or maybe more accurately, the lack of sufficient public opposition).
"Find out just what any people will quietly submit to and you have the exact measure of the injustice and wrong which will be imposed on them." - Frederick Douglass
We already have a government of politicians who explicitly work for their own benefit and for the benefit of their cronies and patrons and against the interests of the common people and who demonstrably lie about it. What on Earth makes you think that they would not, much less could not, take advantage of the simpletons who don't grasp the concept of rights and work to destroy them? Politicians are professionals, and the most powerful of them are necessarily extremely skilled and necessarily extremely power-hungry. They are only waiting for the moment when they can cobble together enough of an appearance of support for shifting power from us to them, at which point they can be counted upon to do so, and to attempt to placate the opposition with the illusion of that power shift being a necessary outcome of the "democratic" process. And for that, they don't need your support or my support - they don't even need "the public's" support. All they need is the colorable illusion of such. If they can only make the claim that they have that support - only point to enough particularly vocal examples of such - they can muster the rest of the sheep around it. Their desired outcome becomes THE outcome as soon as they can claim that it's our desired outcome, which doesn't require the support of the majority - it only requires a lack of sufficiently forceful opposition. If we don't tell them loudly enough and often enough that we do NOT want this, then they can, and will, and do, point to the few who say that they do and claim that it's a "mandate," and enough of the rest will just reflexively fall in line that it becomes one.
So those few who demand that the government be empowered to deny the rights of all - those dolts who can't even grasp the simple concept that a government that's empowered to deny the rights of others will also deny their rights, and once empowered CANNOT BE STOPPED - must be kicked squarely in the teeth with their dangerous stupidity. Because even with the constraints nominally imposed on government by the Constitution (most of which restraints are already ignored anyway, as a matter of fact), their stupidity can and will and does provide the power-hungry few who most keenly seek out office and most predictably come to attain it with all the excuse they need to keep expanding their own power at the cost of our liberty.
Guns aren't even the point, really. Rights are the point. Liberty is the point. Gun ownership is simply an exercise of a right, just as free speech is, or freedom to assemble is, or liberty is, or life is. It's not that government must not be empowered to deny gun ownership - it's that government must not be empowered to deny RIGHTS. That's the point that all too many people can't or won't grasp, and that failure is the thing upon which tyrannies grow.