Let's imagine fascists took over the US federal government. Implicitly, this means at least approximately half the country backed the fascists, so that's how bad it is at the start point. It's unlikely the non-fascists all agree the fascists need to be resisted: a load of them will be too afraid, or think they can follow the laws and systems and prep for the next election, so the non-fascists will be disunified, cautious and indecisive.
This is then where the fascists slowly boil the frog. Gradually soften or erode any forms of resistance: disrupt or disempower resisting state governments, cripple opposition media, suborn the police and courts, establish strong additional power bases outside the state etc (e.g. their own party militia or secret police). Eventually maybe enough non-facsists realise they need to fight... and it's way too late because the systems, networks and resources they needed in order to do so effectively are already gone.