The 'whole US'? No, of course not. If an entire city was against the dictator, then they could do what al-Assad did in Syria: gas the entire city. The problem with an insurgency is that most of the population is either for you or at least pretends to be... so it's impossible to find those working against you.. and that support could be active (by fighting themselves) or passive by refusing to report insurgent activity, for example.

IIRC, the minimum force is 1 soldier per 50 citizens. That's enough for the military to be so prevalent in an area that it would be impossible for insurgents to operate. For the US that would be 6 million soldiers (+ soldiers for the unpopulated areas). Those numbers usually don't include civilian police forces, but even if you did include them, the US military would be millions of people short.

But yes, the insurgency needs support from the people. Not a majority, but a network and general support must exist in order for it to survive.