Well, that process happens in the voter base, not the groups of elected representatives as it does in Europe. Basically an american "party" has to represent int

Well, that process happens in the voter base, not the groups of elected representatives as it does in Europe. Basically an american "party" has to represent interests (at least with lip service as is often the case) of its constituent groups or lose that portion of the voter base, so majority votes for the two parties changes if the interests of the people themselves change, this happens on a state by state as well as a district by district basis. One thing this system prevents that does happen in European style democracies is no confidence and breaking of those coalitions, which cannot happen between elections in the US like it can in many other countries.