At the Volokh Conspiracy, Jim Lindgren notes that Obama has edged ahead in four of the five most recent national polls and suggests the three likeliest explanations:

1. The Republican convention bounce naturally expired.
2. The press’s direct attack on Sarah Palin is working.
3. The Wall Street collapse and the bailouts are turning voters toward Obama/Biden.

Explanation 1 is obvious enough, and Explanation 2 is certainly plausible, given the relentless media negativity toward Palin for the past three weeks.

Explanation 3 is most disturbing. Granted, economics is not John McCain’s strong suit, by his own admission, but Barack Obama certainly has no record in the area. Bad economic news generally disfavors the incumbent party, but Obama must still convince voters that he is the man to be entrusted with the task of recovery and reform.

Let me add a possible fourth factor not listed by Lindgren: Obama has unleashed a wave of harshly negative ads in swing states, and these are having an impact. Obama’s ads flatly call John McCain a liar, and these ads haven’t been effectively countered by McCain’s own ads.