Chauvin appeals and his conviction is reversed by a MAGA judge. Trump pardons the Federal Conviction, sparking riots and civil unrest across the country, giving Trump justification for declaring a national emergency and martial law, which includes an indefinite postponement of all elections.
Putting my tinfoil hat on...: I don't think he needs to declare martial law formally if the Supremes would agree that such riots are sedition or insurrection. He can then continue to use the National Guard as law enforcement, which he is pretty much doing anyway, like he is using Border Patrol to go all over the country away from the borders. As far as postponing elections, I don't think he would do that but again I don't think it's necessary. He can legally declare certain organizations terrorists, like, say, George Soros funds as foreign terrorists, or BLM as domestic terrorists and then look at which politicians they gave money to and force them to give up the money or be in trouble as terrorist support.