If Trump remains in power despite an election that goes against him, this would most likely be an autogolpe and not a coup. A coup refers to a seizure of power by folks not currently in power, usually involving the military. An autogolpe is when an incumbent seizes power. It comes from Spanish because it was a thing in Latin America at various times including rather recently with Fujimori of Peru. Viktor Orban's moves in Hungary have been an autogolpe in slow motion.
To be clear, this is not just me thinking this:
The latest story about how the GOP might seek to subvert the election results by having GOP-dominated state legislatures throw out the popular vote totals in the name of imagined voter fraud and select GOP electors is very autogolpe type stuff.
And if it were too happen, the US would be in a personalist dictatorship, not a military regime, and not, despite some appearances, one party rule. Why? Regarding a military regime, the military would be well out of this and thankfully so. A single party system? Not really since it is more about the personality of Trump and less about the party's ideals, but it could end up being a hybrid. This matters because different types of authoritarian regimes have different life spans and patterns of behavior--see Barbara Geddes's work.
Why do these definitions matter? Because they point to the processes that may tip the US into autocracy and where we need to put our efforts to stop this. If the threat was a coup, we should be focused on the military and deterring or persuading the folks with the guns not to intervene. But if the threat is autogolpe, then we need to figure out ways to prevent Trump from usurping power. Which means putting pressure on these state legislators, on revealing these plans so that opposition increases. Better to resist now than react to a fait accompli.
One of the key things that those in favor of Democracy have is that the Trump people suck at keeping secrets. The downside is that when the Trump folks do things in broad daylight, they look less criminal. Anyhow, none of this is inevitable or foregone. To resist, we need to protect the media who are doing the hard work of reporting these potential plans, and we need to focus on getting as big a win as possible on election day which makes it harder to fudge results.
Since I am not an autogolpe specialist, I hope others who have studied these things can give us some clues. I will say that one of the things that gives me a bit of hope is that aspiring autocrats often lose power when they seek to play with unfortunate (for them) election results.