OK, let's fact check this:
Trump's actual words:
"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..... "
While any form of voting is open to abuse, "substantially fraudulent" implies that the outcome of the election would be in question.
Five states have done voting by mostly by mail already: Colorado, Hawaii, Utah, Washington, and Oregon.
Show me an election in any of those states that was found to be, or even widely suspected of being "substantially fraudulent" in the time since they went to mostly mail voting.
Could mail boxes be robbed? Yes, but since mailing of ballots occurs over a long time frame it would take a hell of an effort to skew the vote substantially. Forged ballots could occur; most of the cases I've heard of involve republicans doing it, incompetently, and getting caught. Illegally printed ballots wouldn't be a thing; they would be obvious fakes.
Face it: Trump is afraid of anything that might increase turnout, because increased turnout hurts him. Most republicans actually like vote-by-mail. Hell, Trump himself has often done it... For example:
President Trump and first lady Melania Trump have cast their ballots in the 2018 midterm elections. A White House spokesperson said Friday that both Trumps voted by absentee ballot i…
thehill.com