Donald Trump has been fulminating about Twitter fact-checking this claim:
There is NO WAY (ZERO!) that Mail-In Ballots will be anything less than substantially fraudulent. Mail boxes will be robbed, ballots will be forged & even illegally printed out & fraudulently signed. The Governor of California is sending Ballots to millions of people, anyone.....
As far as I can tell his objection to being fact-checked is that the mainstream media sources are all his opponents and can't be trusted. Media supporters of Trump seem to agree with him. Tucker Carlson (quoted in the Washington Examiner) argues, for example, that:
I don’t care what Twitter tells you. That’s true. It’s obvious. And by the way, it’s been documented. … It’s not a theoretical risk. Actual voter fraud has happened despite what they tell you all over the country.
There are several claims in the tweet. But the main one is that mail-in ballots will be substantially fraudulent. As far as i can tell nobody doubts that it has happened, as have other forms of voting fraud (or potential fraud with very hackable electronic voting systems which doesn't seem to be a cause of worry for Trump). Carlson supports the Trump narrative by quoting some actual examples (but, since they were all examples where it was detected and led to prosecution, it isn't clear that this explains how the existing ways of preventing it are flawed).
So the claim here is not whether mail-in voting fraud exists but whether it is substantial. Is it?
PS I'm not an american. I only know the rules around mail voting in the UK, which are pretty strict around identity verification. It would be useful if answers provided useful background about US systems, how much they are used, how long they have been used, what the rules are to prevent fraud and how much actual fraud has been detected.