I feel you about the political stuff. Since divorcing isn't realistic, I agree with other commenters that you should just avoid talking about it.
If you must commute together, find something else to do, or ask him if you could just nap in the car while he drives, or listen to headphones, or start some sort of an educational podcast (something nonpolitical/noncontroversial), like the podcasts at iTunes University, maybe? Or delve into a new music genre.
Also, just keep remembering that his negativity is his own, and HAS NOTHING TO DO WITH YOU, and don't let it get to you. You don't have to respond or even really acknowledge it. That you find it irritating is your problem, so just take a deep breath and remind yourself that YOU DON'T CARE. Find some sort of an easy mantra to repeat in your head when you start feeling rage. (Like "cheese, cheese, cheese, cheese.")
Also, you can redirect with adults just as you can with kids. Bring up topics that are of interest to both of you. Be prepared in advance with something. Kill him with kindness, and maybe he'll have a little less negativity.
Good luck!