unlike republican spain, america does not have an officer corps that is fundamentally opposed to the political system of the state.
the spanish republic before franco was young, unproven, weak, unstable, a complete mess all around, and nobody really believed in it: the anarchists on the left wanted - well, anarchy. the communists wanted a soviet aligned dictatorship of the proletariat, aka vanguard party rule. the monarchists on the right wanted monarchy. the carlists on the right wanted monarchy but with a different monarch. and the falangists on the right wanted national syndicalism.
the key takeaway here is that basically nobody in spain wanted the republic as it was, most people wanted something that was radically different, and so they went and fought for it.
this is not the case in america. no serious organisation in america wants a different system of state and government. both major parties are firmly loyal to the republic and its institutions, and so is the military. there will not be a civil war like there was in spain.