Those watching the presidential race in North Carolina -- especially supporters of Barack Obama -- may have felt a little poll whiplash this week.

Since mid-September, a series of polls showed Obama gaining momentum in NC, culminating in a Public Policy Polling survey released this Monday giving the Democrat a 6-point lead.

But then, the polls snapped back. CNN/Time yesterday declared a 49/49 tie. SurveyUSA followed by ging McCain a 3-point edge, 49/46. What's going on?

First, the usual caveats: Each poll on its own is a quick and, in many ways, unreliable snapshot. NC is a tight race, and the polls will go up and down. And with most of these polls, the variance between them is within the margin of error.

But it's also true that some polls are better snapshots than others, and SurveyUSA's poll has two major shortcomings.

First, SurveyUSA's sample of 617 Likely Voters make it one of the smaller polls done in NC -- about half of Public Policy Polling's 1,202 LV sample, which gave Obama a 6-point lead -- so the margin of error is higher.

Second, SurveyUSA appears to be under-counting African-American voters -- and admits that if the number is actually higher, the race could turn out a lot different:

In SurveyUSA's model, blacks are 20% of the North Carolina electorate. However: If black turnout increases, with Barack Obama at the top of the ticket, from approximately 750,000 NC black voters to 850,000 NC black voters, it is possible that Obama wins North Carolina's 15 electoral votes, that Kay Hagan defeats Elizabeth Dole in the US Senate contest, and that Beverly Perdue defeats Pat McCrory in the North Carolina Governor's race.

But according to the latest NC State Board of Election stats, thanks to a surge in new voter registrations this year African-Americans make up 21% of registered voters. That 1% difference between SurveyUSA's model and actual registration statistics amounts to 60,300 voters -- more than half of the 100,000 black voters that SurveyUSA estimates Democrats need.





Overall, SurveyUSA isn't a bad pollster. Some criticize its use of automated poll calls run by computers over people, but experts like Nate Silver at 538.com give SurveyUSA high marks for being one of the most accurate companies.