Ms. Clinton is more popular among voters without college degrees. Meanwhile, Duke University political scientist Brendan Nyhan has crunched numbers that show a college education to be a big predictor for Obama support.
However, Florida's phrasing is potentially misleading -- my results show that average state education levels (i.e. percentage of adults with college degrees) appear to be positively associated with state-level support for Obama. We can't make any inferences about individual-level behavior from macro-level results (this is known as the ecological fallacy).
Sirota, with whom I've had my differences in the past, constructs the graph by plotting Obama's margin against Hillary Clinton excluding "the two senators’ home states (Illinois, New York and Arkansas), the two states where Edwards was a major factor (New Hampshire and Iowa) ... the one state where only Clinton was on the ballot (Michigan) and ... the four states whose Hispanic population is over 25 percent" (CA, AZ, NM, TX)." He concludes that there is a "Race Chasm" of states with black populations of 6 to 17 percent where Obama does especially poorly.
There are several problems with the graph. The first is that the X-axis is not black population, but the rank of black population among the 33 states Sirota considers. While that choice makes the graph look nicer, it potentially obscures what's actually going on in the data. Here's a replication of the Sirota graph with black population on the X-axis:
As you can see, the results look somewhat less clear. Now let's deal with the second problem -- the fact that Sirota treats caucus and primary states identically. As readers pointed out, the U-shaped pattern of Obama support against black population, which I noted in my first post on the subject, goes away when you exclude caucuses, which are (a) public (potentially limiting the influence of race on voting) and (b) favorable territory for Obama given the organizing strength of his campaign. Here's a replication of Sirota's graph with caucus states excluded and black population again on the X-axis:
All of a sudden the racial chasm starts to look a lot less cavernous. An even more useful approach is to specifically consider white support for Obama as measured by exit polls (the real dependent variable of interest) rather than looking at aggregate results. When we do this for the same set of states Sirota considers and fit a linear trend to the data, there's no evidence of a "race chasm" -- sadly, there seems to be instead a negative linear relationship between Obama's white support and black population (correlation= -.47):
When you add back in New Hampshire and the heavily Hispanic states and drop Florida, which I think is more appropriate, the relationship is even stronger (correlation= -.53):
In short, the claim that Obama does uniquely poorly with whites in states with moderate black populations is not well supported.
Update 4/2 10:24 AM: Sirota pointed out a typo in the data for Florida that is now fixed in the graphs above.
Update 4/3 9:53 AM: Josh Marshall links to this post, writing that "I'm not sure this apparent disagreement is anything other than the same point expressed a different way":
[R]acially polarized voting increases with the size of the black population in a given state. That leaves Obama winning a lot of states with few blacks. But once the black population gets into the high single digits, racialized voting kicks in and Obama then can't get enough of the white population to win. Only when blacks approach 20% of the population does the black population get large enough to make up for and often overcome the increased white resistance to voting for Obama.
I'd tend to agree with that story -- apologies to Sirota if I misinterpreted his point.
I should also clarify that, as a commenter points out below, Washington DC is not included in the last two graphs. The reason is that there is no exit poll data available that I've seen on the breakdown of the white vote. If those numbers are available, please send them to me.
Update 4/3 1:06 PM: One way to address Marshall's comment is to consider the rate at which Obama gains black votes and loses white votes as states have more African Americans. This plot graphs Obama's total white and black vote separately as a percentage of the white+black primary electorate:
Obviously, this won't work as well in states with significant Latino and Asian populations, but it's one relatively simple way to look at the data.
Also, a commenter below is concerned that I'm wrongly imposing linearity on the data, so here is an identical plot to the one above with a flexible polynomial instead of a linear fit -- there's some evidence of the relationship flattening out but I'm not sure how much I believe it. There's certainly no "chasm" effect in white support:
Finally, a commenter wondered about the relationship with the South excluded -- here's a plot that also excludes all of the states that were part of the Confederacy (one traditional definition of the South):
Update 4/3 5:59 PM: Here are two more graphs that may be of interest. The first graphs Obama's total white and black vote separately as a percentage of the white+black primary electorate again but this time with flexible polynomial fits instead of linear ones:
The second tries to look at combined white and black support for Obama across the range of black populations by state. See what you think:
(Note: There are so few African Americans in the extremely white states that the exit polls can't estimate the rate at which they voted for Obama. So I put in 85 percent as an approximation to avoid having those states be missing from the graph.)
Update 4/3 8:02 PM: Marshall discusses Sirota's column and my post in a TPM video.