Identity politics has been a subtext (and a often a supertext) of the
race between Clinton and Obama. This is, in effect, the only way to
interpret the age / gender / racial voting patterns so far. Depending
on the underlying demographics, they become more or less pronounced
from state to state, but they are omni-present.

Underlying
this is the following intuition: people like to associate with and vote
for candidates with whom they identify, i.e., people they perceive to
be like them. Obama is fundamentally similar to three demographics:
blacks, men, and the affluent middle class. His campaign, until the
Wright affair came up, was able to make him appeal to all of these
groups, without making his black identity too salient to white men or
white folks of the affluent middle class. The Wright affair made his
black identity salient. While Obama also identifies with the affluent
middle class - just as strongly, only a (very?) small minority of white
people have much familiarity with black culture. Hence, showing aspects
of that culture is scary - it reminds people that Obama, in part, is not like them.

The
campaign had to address this. Obama is affluent, male and black. He is
now explaining to us why this is an asset. In a sense, the takeaway
from Obama's speech is "I am also like you and that's because I have
shared experiences with all of you". It is a complex and nuanced
message - and it will take some time to see if it was well received.

Given
that Obama is the more talented politician, Clinton's only strategy has
been to make Obama either corrupt (didn't work so well), inexperienced
(working a little bit, but not very well), or not like you (working
well). This is why it is good for the Clinton campaign to make the
discussion about race. The more we think about Obama's blackness, the
more white people think "OMIGOD! he's not like me!" - and also why
Obama rushed to change the subject today with his foreign policy speech.

Clinton, for her part, actively pushes identity politics. For example, in her stump speech she often mentions how great it
is that the two Democratic finalists are a woman and an African American. The
reason she does this is because there are more women than blacks - and
hence she is then simultaneously reminding more voters that she is like
them and Obama is not - then the other way around. This is a highly
cynical, scorched earth, strategy.

If
Clinton is nominated, it will be because a plurality (or at least a
perceived plurality) of voters were afraid of voting for someone not
like them. The same holds for McCain in the general. Obama's path to the presidency requires him to neutralize the race
issue.

A footnote: I
recognize and respect that not everyone is influenced by identity politics - but enough are that Obama's losing ground in the polls.