Why does everyone assume midterm elections will be held normally? It's totally obvious that Trump will interfere with them or cancel them altogether.
How do I know that?
Firstly, because he and his henchmen have said so repeatedly - and you should believe them when they say they're gonna do something.
Secondly, Trump trying this hard to be unpopular means he doesn't give a shit about being popular to win votes, because he knows there won't be elections.