I would only not attend if it's a financial burden. Don't try to send a message. It won't be heard. Simply assess if it is no longer worth your time and effort and let him know you aren't interested in upending your life on his behalf.
If you've got the money and it isn't a big deal to take a nice vacation, still attend the wedding. After the honeymoon, let him know you're done with his ass.
Aside: I'm presently evaluating whether my mother and I will maintain a relationship. I'm extremely angry with her for voting this way yet again and I have concluded (based on the day-one exec orders) that she is partially a bad person because of her politics. I still love her, but I've had enough. If you're going to hurt the world with your vote, I may not continue having you in my life.