Earlier quoted context omitted.
And this is specifically on a tweet calling the validity of the election into question. It's blatantly wrong, but he needs his base to believe him when he says the election is rigged.
Hasn't the other side been #NotMyPresient and throwing a tantrum about the validity of the last election? Trump is not a trailblazer in this regard
1. Questioning whether the Electoral College (and its tendency to devalue votes in some states) has a place in the modern US.
2. Questioning whether the election itself is completely rigged (via fraudulent votes).
#1 is the question many liberals have been asking. #2 is thee claim that the entire GOP has been making for years, despite their own investigations never turning up more than a few individuals voting fraudulently (but never systematic fraud perpetrated by the political left, as they claim).