I hope this doesn't break any of the rules. I know it may skirt the line on the no <20 year rule(since it does have to do with two alive political parties), but I think that it (hopefully) falls in the past enough for it to be non-current-political. If not mods please let me know and I will delete or edit the question to fit back into the rules.

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It is a common assertion that at some point after the civil war, the parties "switched." From what I can tell they take it to mean the ideological positions of the democrats became that of the republicans and vice versa. However whenever I ask someone who asserts this (usually to claim that Lincoln was somehow "really" a democrat) I never get a clear answer on what exactly switched about the parties.

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For example, the democrats circa civil war were pro-slavery, and now they (at least in rhetoric) stand up against racism. But that doesn't mean that the Republicans became totally pro-slavery (took on the democrats position); I personally don't consider the average modern-day republican racist. Likewise, from my understanding, the original democrats were proponents of small government, and the original republicans were classical liberal, which also tends to favor markets over governments. Of course both sides tend to take a large bureaucratic state as a given.

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It seems to me more like there was an evolution of ideas so that both parties occupied ideological positions that were different than the one that they had before, but also different than what their counterpart had before. It would seem strange to me that so many people would change their party to the opposite rather than just join the other party. So is there any truth to the switching sides argument, or is it just an urban legend?