I say this, not to inflame, but to ask...Does he want to keep you on the Government Plantation? Does he feel it reasonable to not respect your opinion? You seem informed and capable of making a reasoned decision about who you thought might offer the best leadership to the country and for you posterity. Your father-in-law is presumptuous and unreasonable. You are a grown man. Make your own decisions. If he disagrees, fine...if he react so irrationally it is a problem of his own making. You have been gracious to him with your exposition. That should placate him, but if it does not, shake the dust off of your cloak, back away, and give him all the time he needs to come to his senses.
You are absolutely right in all that you say. I really have trouble conceiving why someone would be pissed off about the fact that I came to a reasoned decision about what's best for the everyone. Even in my most extreme liberal days, I would have always welcome hearing someone with another perspective, and I would never have allowed said perspective to influence my personal relationship with a family member.