Trump is demonizing mail-in voting to get ahead of November - 2020-04-08
There's no evidence that mail-in voting is susceptible to fraud. But there's plenty of evidence that more people vote if given the option to do so by mail, and that's what terrifies Republicans.
Now that voting-by-mail has presented itself as a perfectly reasonable policy response to the uncertainty of conducting an election during a pandemic — the world will not have returned to normal by this fall — Trump feels he needs to get ahead of the curve and demonize mail-in ballots before various swing state jurisdictions start coming to their senses.
Only a week ago Trump, showed his cards on what this is all really about, talking about the stimulus bill — "The things they had in there were crazy. They had things — levels of voting that, if you ever agreed to it, you'd never have a Republican elected in this country again."