After moving upward in unison for several days the tracking polls diverged today with Obama's margin growing to 9 points in the Gallup poll and shrinking to 5 points (from 6 yesterday) in the Rasmussen poll.
For skeptics of tying poll movement to specific campaign events, this divergence comes as no surprise. My own take is that two separate polls, both trending in the same direction for several days, with a likely event to move them that way, adds up to a likelihood that the movement is not mere statistical noise. But then why the divergence today? Here's one item I found on TPM written during the Democratic primary which may explain it:
But in yesterday's sample, Rasmussen and Gallup produced very different results. Why would that be? The obvious answer is that it was Saturday, the toughest day of the week for polling. (Mark Blumenthal has eloquently detailed the challenges associated with Saturday polling at pollster.com, for the curious.) In general this cycle, Saturday polling has hurt Obama. It tends to undersample those who are out and about, and has a particularly large effect in robopolls, where many respondents hang up when they're busy. Gallup has a much more robust methodology, and though the poll is fairly new, we've seen much less variation between its Saturday and weekday samples that with Rasmussen.
If this explanation is right, it may explain why adding yesterday's data to the 3-day Rasmussen average caused the margin to drop in that poll. What is your take?
Update: Via Hopeful in NJ in Quick Hits, a new poll finds Obama up 12.
