Let's take as a starting point this list of methods:
http://www.heritage.org/election-integrity/heritage-explains...
(I realize that's from the Heritage Foundation - I'm not particularly conservative - it's just the first decent list I came across while Googling.)
Let's take these 3:
"""
False registrations: Voting under fraudulent voter registrations that either use a phony name and a real or fake address or claim residence in a particular jurisdiction where the registered voter does not actually live and is not entitled to vote.
Duplicate voting: Registering in multiple locations and voting in the same election in more than one jurisdiction or state.
Fraudulent use of absentee ballots: Requesting absentee ballots and voting without the knowledge of the actual voter; or obtaining the absentee ballot from a voter and either filling it in directly and forging the voter’s signature or illegally telling the voter who to vote for.
"""
Ok. Pretend I am a hostile nation-state. I can find a list of registered voters online (from some hack, because in 2020 PII is everywhere). I identify a number of key districts in the upcoming US election, and the names of some individuals living in those districts. I acquire some mail-in ballots, and cast votes using those names and details (say, SSNs). I'll just ignore whether or not the person has already voted. (To summarize: my hypothetical attack uses methods (2) Duplicate voting and (3) Fraudulent use of absentee ballots above, not actually method (1) False registrations.)
Is that so hard to imagine doing? I don't think so: particularly not for a nation state.
"Voter fraud" can be when a particular person votes twice in an election -- that's possible but not worth the risk for the individual. In this scenario there's not much real risk to whoever is carrying out the attack.
It doesn't even really matter if you get "caught" (detected). (Getting "caught" (punished) is hard to imagine.) The goal is just to undermine the election. To win, all you have to do is throw the result into doubt.