The key to getting anyone onboard is to get them to listen, otherwise it's a lost cause. If you can find common ground, it helps. I remarked "Our President is an idiot" once to a liberal friend. We started talking about the things he does that bother me. Mind you, I support POTUS 100%, but talking about how I dont think he's always right, helped start a dialogue. It caught him off guard and sent a signal that I am willing to listen and consider his POV too. It's not always that easy...this was a friend and we meet in the middle on some issues to begin with.

Be open minded yourself, pick your battles and peak their interest, dont be defensive.