Political parties being made of people, I'd expect their ideological alignments to shift no more quickly than their constituents';

Political parties are also often made of factions. The outcomes of their power struggles might significantly change the party's character.

In general, I suspect we need to be clear about when we are speaking about human universals and when we are speaking about politics in a given country. The US political parties are not the same as, say, the European political parties and I would expect the political behaviour (e.g. party loyalty) to be noticeably different between the continents.

In particular, under a two-party system the opportunities for changing party allegiance look to me much more limited compared to under a multi-party system.