Actually, you're on to something, there. Trump couldn't stop the election, short of declaring martial law and deploying troops to every polling location in the country. But it wouldn't be nearly as hard for him to stop the election in Portland. Of course, he probably doesn't care about Portland, but if he declared martial law in, say, Detroit, Philadelphia, Richmond, Milwaukee, Austin...Goat, the mayor of Portland who is responsible for law and order in his community found a federal force in his community thanks to our president.
After declaring a national emergency our president used funds earmarked by Congress for specific purposes to be used for his wall.
Upon declaring a national emergency who is to say our president will honor laws and precedents regarding elections which had formerly been the domain of the states.
Not saying he can pull it off. Just saying he might try.
Not that I think he's actually going to do that, of course, but if he truly wanted to steal the election, that would be the way to do it.