Could a citizens' militia possibly be effective against an attempt
to install a dictator?

Indeed, a citizen militia could be effective against the illegitimate attempt to install a dictatorship in the country through military force. In fact, one of the main limitations that the rulers in the United States have to fall back on authoritarian or dictatorial governments, in addition to the checks and balances system, is the constitutional right that American citizens have to own weapons, inserted in the Second Amendment. Thus, the rulers know that in the event of an illegitimate attempt to gain power, the American people could quickly organize in defense of democracy and civil liberties since they have the necessary elements to do so. That is why the organization of a citizen militia would be perfectly possible and effective in this situation.