Some say that people adopt ideological positions because they match the identity they have chosen, others that real shifts in what people believe are driving them toward identity with their parties. Thomas Edsall has an interesting round-up of these views in the Times. On the one hand:
There is a growing body of work showing that policy preferences are driven more by partisans’ eagerness to support their party rather than considered analysis of the pros and cons of opposing positions on any given issue.and
What if, to some significant extent, the increase in partisanship is not really about anything?I can certainly think of examples where people in both parties have adopted strong positions in indifference to the evidence, or in the complete absence of evidence. So I think this happens. But on the other hand I think some issues, such as increasing concern about inequality, have a resonance that goes beyond simple partisanship.
So there is also this view:
Policy and ideological differences are the primary drivers of polarization. Democratic and Republican voters today hold far more distinctive views across a wide range of issues than they did in the past. And it is among those Democrats and Republicans who hold views typical for their party, that is liberal Democrats and conservative Republicans, that dislike of the opposing party is strongest.But of course that correlation could be driven from either side.
I found the recent election in Kentucky to be an interesting look at American partisanship. Republicans won the legislature and most of the statewide offices, but it looks like the Democrat has won a narrow victory in the governor's race. Voters being quoted in the media said they just didn't like the Republican governor's angry attitude. So while partisanship is strong it can still be overcome by other factors.