Are there any political issues, such as abortion or capital punishment, that are so fundamental to your core values that you could not respect and/or trust someone who held a contrary view?
I don't think so, but it would really depend upon why that person held that particular view. If someone is able to explain themselves calmly and rationally, and they don't lose respect or trust in me for disagreeing, I don't think I can imagine any legitimate opinion that is totally Unforgivable.
All of those qualifiers, however, might need some elaboration. First, I can't respect anyone (or, at least, anyone's opinion) who doesn't know why they have a particular opinion or view. If you think or feel or believe a certain way, "just because I do," you probably shouldn't expect people to respect that opinion. An opinion is not, by virtue of being an opinion, self-validating. Try some introspection, for pity's sake. Or, better yet, do some research.
Second, and this sort of goes along with the first point, I find it very difficult to respect someone who immediately becomes angry or irrationally vehement when a particular point or subject is brought up simply because it is being made a topic of discussion. Insisting that something shouldn't be discussed, or assuming that the discussion itself is insulting because it calls something you consider fundamental into question, or taking the discussion personally will severely reduce my opinion of you. In that scenario, the only reason to believe what you believe is to have a comfortable world view that makes you happy, which does not lead me to have a great deal of respect for what you believe.
And finally, it must be what I refered to a few paragraphs ago as a "legitimate opinion." That's very vague, so I'll qualify it through example. The idea that fetuses deserve every right that fully formed humans do is, while extremely disagreeable to me, a legitimate opinion. You can argue that. Life is one of the great mysteries of Human existence. The idea that the Holocaust never took place, that people who are overweight should be put to death because they're a drain on our food supply, or that the HIV epidemic is God's wrath on the lustful is NOT a legitimate opinion. And yes, I have heard those made into political opinions.
So... yeah. That's that, then.