I think your DH may have felt caught in the middle.....on the other hand, he could have offered you 2 had to be somewhere when it started to get heated.
I have learned to be very careful about talking politics. I used to get fired up about that stuff with others and enjoyed a good debate, but not anymore. I can talk to certain people about it but I know what I think and can keep my opinions to myself especially when the conversation involves someone who loves to rant.....if we can't discuss our views without hositility, then I just don't. I know that sometimes people's reaction take us off guard, which is why I'm really careful about who to discuss political topics in-depth with.
He is still your BIL and will be around......so next time just avoid the political topic and act as if nothing ever happened and if he brings it up just say, "you know we can't talk politics (with a smile and a hug), we'll just have to agree to disagree on those subjects.......now who wants ice cream?"
then stick to it......you can still vote for the person who best represents your values and it is your right not to share whom you vote for if you choose that.
We're all human and have more in common than not, and I hope to focus more on that these days.