Bottom up or Top down?
A system change either to open fascism or communism bottom up is difficult, and the country adopting it would probably not be the US as we know it.
The most likely way is a radicalization in a huge economic crisis leading to very different leading fractions in various parts of the country. Therefore, several states (or city vs state governments) couldn't accept the national government anymore, and would secede (or declare it deposed). [Think civil wars on state level - with a national government unwilling or incapable of putting the lid on it]. So the US undergoes a period close to failed-state ism. Eventually, one regional group / or a coalition of regional groups, gets predominant in this civil war, and proclaims a New United States (or something called differently) at gunpoint, re-unifying most of what used to be the US.
The other approach is that a person or group that is - or becomes - deeply convinced that such a regime is a necessity actually ends up on top of the national government, but without the public noticing it. Then, they exploit a HUGE crisis, reinforce it to become an absolute disaster, and blame the disaster successfully. Under maximum hyperbole, the citizens are then called upon to "fight the imminent danger", take action against "traitors / enemies of the [American] people". Once mob rule starts, congress will approve "urgent emergency measures", and mob gangs will suppress coordinated opposition. The government then sets up a (nominally bi-partisan) coordination committee[CC], with a lot of branch offices, that try to direct the mob rule. Where it gets out of hand, the army will be sent in to "restore order". After a while, elections are held on schedule, with both Republicans and Democrats running only candidates approved by the CC ("true patriots"). After that, a constitutional amendment (or a brand new constitution) should not be to difficult to pass - Congress and state legislatures are filled with loyal regime supporters.