Trump doesn't oppose vote by mail. He's voting that way from his fraudulent voting address. And encouraging other Republicans to do so.
And if that doesn't work it's just a pretext for Trump and Barr to contest the election when that's not even a thing.
He has plain-brand Republicans defending his assault on the post office because they hate the post office, too.
But this an obvious attempt to steal an election where he knows stealing is his best option.