Sometimes the assumptions made are really correct

In the Democratic primary race, Mr. Obama and Ms. Clinton are essentially tied in terms of how many votes each has gotten. In various sub-populations their support is quite variable: Among African-Americans, Obama has been getting 90% and among white women, Clinton is getting 2/3 of their vote.

The assumption the press seems to be making is that Democratic voters are following racial and gender lines when deciding who to vote for. Well, not all of them are: We can reasonably expect each candidate to get 50%, since on average that is what they are getting. However far the total skews from that can be attributed to "identity" voting. So, in Obama's case he gets 90% of the AA vote. So, 40 of his percentage points are coming from people voting along racial lines.

I expect this is nothing new: For instance, who would be surprised if JFK got higher percentages among Irish and Catholics, then he would have had he not been an Irish Catholic?