I think it hinges on their (Trump, really) confidence on winning in November. No reason to do it now if you're sure you're going to win. No reason to worry about possible repercussions in the form of lost voters if you're sure you're going to lose.
It would be a perfect game theory move to do it, no merit in holding out.
The election will likely be contested. When some strategically selected counties in Florida start anulling mail-in votes the Republicans will be glad to have a person who will dispute in their favour.