The previous staff post discussed one element of Barack Obama's new-voter strategy, his historic strength among young voters. There's some fascinating new evidence about the magnitude of his appeal to another element, African-American voters.

Via Jim Galloway of the Atlanta Journal-Constitution, we learn that state election officials (from data required by the Voting Right Act) estimate that nearly 40% of early voters in Georgia as of last week were African-Americans. Black voters represent 29% of registered voters in the state, a figure that's up sharply this year because nearly half of new voter registrations this year have been among African-Americans.

At fivethirtyeight.com, Nate Silver uses these numbers to conduct a very useful demonstration of the potential impact of two interrelated phenomena: potentially historic African-American turnout combined with margins for Barack Obama that are unlike anything seen since Reconstruction. (This second factor, the almost universally ignored phenomenon of African-American swing voters, is something I discussed a while back in the context of Virginia).

Stipulating that Obama will carry 95% of the black vote (which is what most national polls indicate) and 30% of the "nonblack" vote (whites, Hispanics and Asians) in Georgia, Silver shows what various levels of black turnout would do to John McCain's relatively strong lead in recent polls of the state:

[S]uppose that black and nonblack voters each turn out at the same rates as they did in 2004, but that we account for the increase in black registration. According to our math, John McCain's 7.0-point lead is now cut to 4.9 points.

But that is probably too conservative an assumption. Newly-registered voters -- and nearly half of Georgia's newly-registered voters are black -- turn out at higher rates than previously registered voters. In addition, one would assume that the opportunity to vote for the first African-American nominee might be just a little bit of a motivating factor for black voters. Suppose that African-Americans represent 29.0 percent of Georgia's turnout, matching their share of active registrations. Using the splits we described above, McCain's lead is now cut to 2.3 points.

Even this, however, may be too conservative. For one thing, the registration window in Georgia is not yet over ... it concludes today. The statistics I cited above only reflected registrations through September 30. There is typically a surge of registrations in the final few days before the deadline. In 2004, Georgia's active voter rolls increased by about 150,000 persons in the first four days of October, before the registration deadline closed. That was more than they'd increased in the entire month of September.

So suppose that by tonight, black voters have increased to 30 percent of Georgia's registered voter pool. Plugging that 30 percent number in, McCain's advantage is a mere 1 point.

Looking at the early voting figures, Silver concludes that "Barack Obama is winning Georgia right now."

Now I don't think Nate Silver, or anyone else, is ready to actually predict that result, particularly since the Obama campaign has taken Georgia off its target list of battleground states. But as Silver notes, the evidence from Georgia may be important in terms of what could happen in closer states--VA and NC, certainly, and perhaps FL and even IN--with sizable African-American populations. A significant surge in African-American voting levels, combined with historic margins for Obama, could be decisive on November 4, and also represent bad news for down-ballot Republicans in those states.