> Imagine if the US suddenly had a dictator
This is why we have the second amendment. And the constitution as the thing to which office-holders swear allegiance to rather than to "the party" or "the president".
I always find this sentiment a little silly - if the US President went in to full dictator mode and had the support of the military, do you really think a militia of armed citizens would be anything but gnats against the windshield of the United States Armed Forces? And if s/he did not have the support of the Armed Forces, it would not be a very effective dictatorship and you would not even need guns for a rebellion. I truly do not get it.
People have this vision of a civil war, some all-out armed conflict with winner-take-all. Looking at any country with a history of armed rebels, you see a long-term political standoff that occasionally runs hot. Ruby Ridge and Waco are small scale examples in the US. On a larger scale these types of movements actually can accomplish quite a bit. I'm not condoning it by any means, I think it's completely nuts, but I don't think it's so far fetched. If you get enough people with enough political will they can accomplish a lot, armed or not.