Blog mentions are, like search terms, something of a special case. They probably show the least conclusive representation of candidate support, because there is no way to assess whether such mentions reflect a preponderance of positive or negative evaluations of the candidates. Anecdotal evidence suggests that negative mentions of candidate A by blogs supporting candidate B, and vice-versa, are very common indeed.
Probably as a result of this variability in the motivations of blog posters, the presidential candidates are broadly similar in their number of blog mentions over time, with a slight overall advantage to Obama, steadily diminishing over time and ultimately giving way to more recent advantage for McCain. Throughout, there is a great deal of day-to-day variation that is likely attributable to blog coverage of particular campaign events, both offline and online.
In a striking example of this news-driven nature of blog postings, both presidential candidates were completely eclipsed, beginning in late August, by blog mentions of the their respective running mates -- with relative-newcomer Palin, in an echo of the spike in popularity of her name as a Google search term, being far more widely mentioned than Biden during this phase.
Here are Technorati numbers as charted by techPresident.com.
This pattern also tends to echo, perhaps somewhat more surprisingly, the way in which the traditional media cover campaigns, with more-or-less equal time devoted to each candidate over a long span, but with week-to-week coverage favoring the "newest" story, and with plenty of apparently event-driven day-to-say shifts in the balance of coverage. Perhaps the real news here is that patterns of coverage in the blogosphere and in the traditional news media are not so different after all. But that is a topic for another article.