I certainly agree JB, but I also think
@fatguyinalittlecoat brings up an interesting point. There are definitely people in each party who feel that the values of the other party are something truly immoral and evil and go well beyond a mere difference of opinion. Similarly there was a time where I'm sure communists and nazi's were walking around and people were saying that it was nothing more than a different opinion.
I think abortion is a huge issue on how this can play out. There are plenty of Republicans, especially heavily religious ones, for whom it goes well beyond a simple difference in opinion. They see it as literal child murder and hence anyone voting in favor of someone that supports it is condoning baby murdering. Obviously if you were at a point where you truly believed it equated to baby murdering, then finding out someone condones baby murder is going to change your opinion of them a lot more than "oh well they think differently than me on that issue I guess we'll agree to disagree".
I think some democrats feel the same way on gun control, that people/a party turning their backs on even discussing/researching gun control in the wake of major school shootings is truly an evil act, and that hence anyone that supports it is truly evil.
The same can be said of welfare ("you're literally stealing from me" vs. "you're literally letting people die"), religion ("forcing me to obey laws based on a religious text is evil" vs. "our nation is founded on Christian Principles and not following my holy book is a slight against God and represents evil"), etc.
Obviously these people lie at the more extreme ends of each party but there are plenty of them out there, and all it takes is someone being on the extreme end of one issue to see the party on the other side of that issue as truly evil.
If someone thinks it's okay to own slaves that is very much going to change your opinion of someone to an extreme yet a few hundred years ago if we were having this same conversation we would be saying hey now, just because Southern Democrats have different political beliefs than you it doesn't make them bad people.
I'm not saying any of today's issues are similar to that but there are certainly people out there that believe some of them are and view a political party that condones abortion as every bit as evil as one that condones slavery. Even as someone that is heavily pro-choice I can understand how some people feel that way.