If there's no Dem running, I'd say work for him. Consider it as a way of working against the guy you really don't want in.

You'll begin getting the experience you'll need if you want to pursue politics - as a candidate or strategist. You won't be the first guy who's worked for both parties. Who knows, maybe the guy you work for will see the things you believe in just a little differently. If, that is, he's open minded and listens to someone who's not a sycophant.

The other side is, you might begin to understand him and his beliefs in a different light.

I've worked as a minor league advisor to candidates. I didn't shape policy but sometimes I gave them a new insight or a different way to think about an issue. It also gave me a lot of insight into how the "game" is played. And the real character of the players. In my case it mostly made me cynical.

Good luck, whatever you decide.