My dad is constantly talking about how bad his marriage and divorce was with my mom which was 37 years ago. Suggestions?

I have told my dad I will leave any time he says anything negative about my mom.

Does he have dementia? Will asking him to stop have any effect? If it will, try saying, "I love my mother. Please stop insulting her. It hurts me." If he can't stop, walk out of the room the moment he starts. Keep yourself sane.

If it was me, I wouldn’t listen at all. The minute he started I’d leave, and though you don’t control others, you can encourage them to do the same. Dad may not be able to change this particular fixation but it doesn’t mean you need to listen to upsetting information over and over again. I wish you both peace

My guess is that in his mind this just happened so this is why he is obsessing over it (I say this as a person who went through a bad divorce...it is life consuming at the time).

There is nothing wrong with you telling him you won't discuss and will leave if he will not stop.....and do it if he does not comply.

I would just cut him short and say "I don't want to hear you say awful things about my mother...let's change the subject or I'm leaving."  He will probably do it again and again so you will have to repeat yourself time after time, but he will see that every time he does it, he will hit a brick wall.

Every time he goes there stop him and tell him if he continues you will be forces to leave, tell him it hurts you because if he had not married your mother you would not exist. If he stops fine stay and visit, if he continues get up and go.

My mom did this right up until it became apparent she had dementia...now that talk stopped. I heard too much personal stuff for many years..I think early stages of dementia caused a lack of insight and she felt ok with talking about her personal issues with my dad. what we did was walk away when it started or I would tell her “mom he was my father,,,I love him for that. What he did as a husband was between the two of you“ ..mine parents were married 52 yrs and she stayed for it all!

You have a lot of good advice here. I will add my two cents - control the conversations yourself. Start with: Hey Dad, do you remember when... and add one of your or his favorite family stories. Any time he brings up Mom jump in louder and happier with another family story, or something about one of his siblings, or his second wife and a favorite holiday experience. You can tell the same stories over and over. Try to have enough of them that you can alternate if you talk to him every day. That means you need 7 stories to engage him with. If he has dementia, which it certainly sounds like, he might actually enjoy this.

Give him one warning if he starts on your Mom, and if he does it again get up and walk out.

Up and away whenever he starts would be my advice.
I was successful in getting my aunt to not discuss unpleasant “stories” by insisting she stop.
Shes 93 and has dementia. She learned that I would not put up with it. My theory was each time she said it, she reinforced that unpleasant memory. I did not argue with her. I did remind her that one of her stories happened to her friend and not to herself.

She wanted me to stay with her and visit so she would stop but I do know that she’s told these stories to her aides so I can’t say she forgot them.
This may or may not work with your dad but might be worth a try.

Look up rumination. I think this is what your dad is doing.

I like the idea of bringing a topic to discuss before he gets started.

I would also like to gently suggest that you might benefit from a bit of therapy yourself as this had to have been a difficult childhood if your parents were so unhappy while you were growing up. This may be an opportunity for you to heal from that trauma. Staying and listening to it may be very harmful to not only your present but also future happiness.
Do take care of yourself. I’m sorry for this difficult time.