Political identity is deeply fundamental to people

To some people. Not to all people.

I may be falling victim to the typical mind fallacy here (my identity is not linked to any political group), but the latest Gallup poll says: "Forty-two percent of Americans, on average, identified as political independents in 2013, the highest Gallup has measured since it began conducting interviews by telephone 25 years ago. Meanwhile, Republican identification fell to 25%, the lowest over that time span. At 31%, Democratic identification is unchanged from the last four years but down from 36% in 2008."

The same link shows a graph for declared political affiliation and I it doesn't look likely to me that the shifts in political allegiance are solely due to some cohort dying off and another cohort growing up to be counted in the poll.