Ohio was somewhat of a key state in the 2012 election. I finally got a chance to look at the vote counts and exit polls in detail.
B. Obama M. Romney White (79%) 41% 57% Black (15%) 96% 3% Hispanic/Latino (3%) 54% 42%
Note that 3% of the respondents did not fall in any of these categories, and they are omitted from analysis.
These numbers impute a vote share of 48.41% to President Obama and 46.74% to Mr. Romney. Obama's advantage among Hispanic/Latino respondents translates to 0.36 percentage point of that margin. Romney's advantage among White voters translates to a 12.64 percentage points of his support. Obama's overwhelming advantage among Black respondents, however, offsets that by 13.95 percentage points.
According to the 2008 exit poll, that year 83% of the respondents were White and 11% of the respondents were Black.
Now, in 2008, 5.6 million votes were cast in Ohio. This year, 5.3 million. Romney got 80 thousand fewer votes than McCain did in 2008. Obama got 236 thousand fewer votes than he did in 2008.
11% of the 2008 Ohio votes is roughly 617 thousand. 15% of the 2012 Ohio votes is 794 thousand. That is, there were 177 thousand more Black voters in Ohio in 2012, representing an increase of roughly 29%, than in 2008.
That translated to about 164 thousand more votes for President Obama from Black voters in Ohio.
Similarly, there were 474,619 fewer White voters in Ohio in 2012 than in 2008.
According to the 2008 Exit Poll, President Obama won 46% of White voters who made up 83% of the voters in 2008, i.e. about 2.1 million of them. This year, he won 41% of White voters who made up 79% of the voters, i.e. about 1.7 million. That translates to about 427 thousand fewer votes from White voters for the President.
According to the Census Bureau's American FactFinder, there are about one million African/Americans over 18 in Ohio.
The numbers thus seem to imply that almost 80% of them voted in this election.