Greg, you may be surprised to find out that saying, for instance "My neighbour is an arsonist.  But that's just my opinion" is not a defence to a charge of criminal (or even civil) defamation.

When anyone makes allegations of criminal conduct against a group of people, they have an obligation to provide evidence of some sort.  Trump didn't.  If there have been charges laid (still only allegations) they must not be more than a handful or they would now be front page news on Breitbart.

It wasn't an opinion.  It wasn't an alternative fact.  It was a lie.

Your statement: ". If people stopped harping on about the popular vote, he wouldn't have to try and defend himself" does not justify a lie.