UDPATED (5/2, 12:29pm): I have replaced the earlier American Research Group survey with the one released today and added the Downs Center poll that was just released.
I spent 5 years in Bloomington, IN while I got my PhD from Indiana University so I can tell you first-hand that the state is not the most racially diverse place. Yet, as I poured over the polling released over the past week in advance of the Indiana primary I came to realize that the relatively small African American population in Indiana may play a fairly significant role in the Democratic nomination race.
There have been several polls released out of Indiana during the past week. Surveys by Howey-Gauge and Research 2000 put Obama in a statistical dead-heat with Clinton, while surveys conducted by Public Policy Polling, Survey USA, TeleResearch, and American Research Group. One explanation for these divergent results may be the assumptions that pollsters are making about black turnout in the state. The figure below plots the percentage of each polling firm's sample that was black and the margin for Obama.
Note that most of the pollsters are using samples that are about 10-12% black, while Howey-Gauge stands out for its sample which was one-fifth African American. (Unfortunately, I was unable to track down the racial makeup of the Research 2000 poll, which also showed a statistical dead-heat). It is not surprising to find that the one survey organization who assumes much higher black turnout would be the one organization showing a small lead for Obama (rather than a significant deficit).
But which polling firm is making the right assumptions? African Americans make up about 9% of Indiana's population, but what percentage of the Democratic electorate might they comprise? Unfortunately, the Indiana presidential primary has not mattered for quite some time, so there is really no baseline for understanding what African American turnout would be in a typical presidential primary. Therefore, the best I can do here is use exit polls from the 2004 general election to get a sense of what share of the Democratic electorate is comprised of African American voters.
According to the exit polls, about 17% of Hoosiers who voted for John Kerry in 2004 were black. This figure is closer to the Howey-Gauge estimates than those being used by the other pollsters. On the other hand, blacks made up 15% of all Indiana voters who identified as Democrats in 2004, and 13% of those identifying as Democrats or independents. This figure would fall closer to the what the other pollsters are estimating for African American turnout in this primary.
Ultimately, we don't really know what to expect in terms of African American turnout. If it is around 10%, then it looks like Clinton may win by 5-10%. If black turnout approaches 20%, then Obama may be able to pull out a narrow victory that would go a long way to ending the race. Could African Americans in Indiana really play such a decisive role? Possibly.