Hm. I don't think there's a cut and dry response here but I'll do my best.

I have family who (as far as I know) do not post racist things but are opposed to me politically. I have had similar strife about political matters with them.

The best thing to do with them (in my scenario) is to ignore it. I don't get involved in Facebook debates anymore, and I know if I did, I would probably be right, but I don't do it. It affects my mental health negatively to get in fights with people about stuff.

However, if you feel like this is going too far, perhaps you could have an offline discussion with your significant other about how this is making you feel. I don't know if you think that would be possible or not, but it seems like this is affecting you significantly.

Then, perhaps if this discussion with your significant other goes well (or at least is constructive), then you will have a plan as to how to approach this in the future.