Trump claims mail-in voting leads to voter fraud without evidence

The repeated editorial comment by the media that Trump fails to present evidence to support his claim that mail-in voting leads to voter fraud or that the claim is unsubstantiated is a shame and a sham. Any sentient human knows fraud occurs. It also understands that mail-in ballots are more prone to fraud than in-person voting. Moreover, substantial evidence exists that fraud occurs. See this report by Politico, this database complied by the Heritage Foundation, this academic study of voting by non-citizens, and the recent problems with and abuses of mail-in and absentee ballots in New York, North Carolina and New Jersey. 

Commentators can argue about whether fraud is widespread or not and how many outcomes it affects. The argument is constrained because we don't know how much fraud occurs. We don't know because we don't examine many elections or ballots closely. Prosecutors have little reason to investigate unless the fraud is egregious or obvious. Moreover, if the "right" candidate won the prosecutor may not want to investigate. Not finding fraud when no one is looking for it does not prove that fraud does not exist.