Is this true? I don't think it is, and I think that has been borne out in several of our most recent presidential elections. In the last election alone, Clinton won NH by a mere 2700 votes, for instance.
OP edited their post, but they were talking about methods to change maybe one or two dozen votes at a time. The minimum number of votes that you would need to reverse the 2016 presidential election was 107k. That was if you had perfect polling going in and knew exactly what votes to change. In practice you would likely need to change several hundred thousand votes if not over a million. This process would need to be done in multiple states all over the country. You would need a large distributed team of people all committed to defrauding an election. That isn't a smart plan for a conspiracy.
Online voting could be vulnerable to such an attack but paper-based voting such as is used by mail in voting is very low tech and distributed. hacking and fraud are hard to scale in that case.
