Can my employers force me to go to a motivational speech by the opposite political party?

No, this is illegal, of course they can then find some nit picky untrue thing to fire you for. If it was me I would contact the ACLU of Florida immediately, then if you get fired you have doentation, also this is simply wrong, but I know too how hard jobs are to come by now. Good Luck P.S. If you do get fired sue the sh*t out of them.