Yeah see, this is how it gets wearing, the lies, the mis-directions, the omissions of information.
No one, not Trump, no Republicans are against mail in voting. It has been used forever.
The normal process has been that the voter requests a mail-in voting form by filling out the appropriate application with all their identifications etc. It gets sent to them, they fill out their vote and post it back. Easy-peasy and more secure. If the voter does not receive the forms (perhaps due to theft from their mailbox) then they can alert the authorities and those forms can be annulled and new ones issued.
However, the proposed process by the Democrats is mass mailing of voting forms whether voters have asked for them or not. It's obvious how many ways this could be taken advantage of but another issue with it is that voter databases are not up to date. People have moved house, people have died etc.
Anyway, we can argue until we're blue in the face, it won't change anything. If it turns out that Democrat states go ahead and perform mass-mail out despite all the issues with that then of course it will be challenged in court. Ultimately (as we saw in 2000) such cases are decided by the Supreme court so sad timing that RBG passed away recently isn't it?