Earlier this year, Fox News switched its polling firm from Opinion Dynamics to Anderson Robbins Research (D)/Shaw and Company Research (R), a joint bipartisan collaboration.

This change has produced a shift in the Fox News poll. Previously, it was usually on the right hand side of the RealClearPolitics’ average of Obama’s job approval, along with Gallup, Quinnipiac, and Rasmussen. Lately, however, it has fallen on the left, mixed in with polls from major media outlets.

One reason for the shift might be the partisan balance in the poll. Opinion Dynamics usually found a pretty even party split, with Republicans occasionally taking a lead. Since Anderson Robbins/Shaw and Company started polling, however, they’ve consistently found a Democratic partisan advantage that recently has grown quite large.

We can appreciate this by examining the party spread in the final two Opinion Dynamics polls with the five polls conducted by Anderson Robbins / Shaw & Company poll to date.

Notice that in the last two Anderson Robbins/Shaw & Company polls, the Democrats have opened up a wide lead on the Republicans (and the 4/28 poll pre-dates the killing of Osama bin Laden).

From February to early April, the two parties were closer together in the new poll, but there is still about a three-point shift in the party balance relative to those last Opinion Dynamics polls. Additionally, the share of independent voters in those February, March, and April polls declined by about 2.5 points. Since independents don't back Obama nearly as strongly as Democrats, the president's numbers have probably been boosted by their decline.

Because partisanship is the best predictor of feelings about the president, these changes are probably sufficient to explain why the poll overall has shifted toward Obama this year.

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