The African-American population share is the key determinant of early voting behavior. In states where there are a lot of black voters, early voting is way, way up. In states with fewer African-Americans, the rates of early voting are relatively normal.
This works at the county level too. In Cuyahoga County, Ohio (Cleveland), which about 30 percent black, twice as many people have already voted early as in all of 2004. In Franklin County (Columbus), which is about 18 percent black and also has tons of students, early voting is already about 3x its 2004 total.
That might explain what's going on in Texas as well. For example, Dallas County has a population that is about twenty-one percent African-American, about eleven percentage points over the total in Texas. And in Dallas County, nearly 90,000 more people have turned out to vote in the first week of early voting, and estimates are that sixty percent of the total vote could come in early voting this year (over a normal total of fifty percent.) More tellingly, early voting in precincts that are predominantly African-American is way up, as much as three times as much in one precinct in Dallas County. In Tarrant County, which has a population that's about fourteen percent African-American, early voting is up by about by about 70,000 votes after one week of voting. Harris County, which has a population that's about nineteen percent African-American, is seeing early voting totals that are roughly double the 2004 campaign.
Of course, short of knowing exactly how many African-Americans have turned out to vote, one can only speculate the extent to which African-American turnout is driving early voting in Texas, especially since early voting by Democrats in generally is up hugely. Still, the trend of huge African-American turnout that I and my co-bloggers witnessed first-hand during the primary, continues strongly. Nate Silver thinks this trend isn't being factored into some polling, such that Obama could have a bigger lead in states with large African-American populations than we're being led to think thus far. And right here in Texas, Harvey Kronberg thinks this massive turnout-and a likely rout of Republicans in Texas bellweather Harris County-presages good things for Texas Democrats on Nov. 4th.