We can however at least sketch the relationship between online and offline opinion by comparing some of the more reliable Web-based measures of candidate popularity with a more traditional metric of popular opinion. For the latter, a common choice is the .

The Gallup Daily tracker is a three-day moving average. We took this snapshot on the morning of Oct. 20, so that it would cover interviews from Oct. 16-18 -- the first three days after the third and final presidential debate.

Right away, we are struck by the fact that the poll portrays a much closer race overall than do most of the online measures of candidate popularity. For instance, Gallup shows McCain in a virtual tie with Obama in the second half of August -- something none of the online measures depicts. Apparently, there is still some general sampling bias in using the universe of Internet users as representatives of the population at large. If Gallup is right, the online measures as a whole appear to overstate the degree of support for Obama relative to McCain.

If we look a little more closely, however, we do start to see some interesting correlations between the online and offline metrics. For example, after regaining his more-typical lead at the very end of August, Obama then was losing it again on Sept. 7, with McCain's advantage widening to 5% over the next couple of days. This, of course, is our familiar "pinch" in the trend lines, which we encountered in almost all of the site traffic metrics, and which immediately follows the Republican convention in the first week of September. In the Gallup poll, to be sure, it is a crossing of the lines rather than a pinch, since the gap between the candidates was never as wide as it was in the Web-based metrics.

Finally, the same surge in support for Obama after the middle of September -- that we noted in the Quantcast and Alexa numbers as well as in the Intrade markets -- shows up clearly in the Gallup poll. With the exception of a brief tie on Sept. 25, Gallup's story since mid-September has been one of Obama pulling away, reaching a new campaign high of 11% on Oct. 8.