In the month of May, Gallup found a small shift in the proportion of Americans identifying as Democrats, with 45% saying they prefer Dems versus 39% who favor Republicans, up from 44D-45R in April. As it happens, it's the best Team Blue has done in a while:
In fact, Democrats haven't seen this big a margin since October 2009, when they held a seven-point advantage. Though the change from last month might seem small, Gallup notes:
These results are based on more than 30,000 interviews conducted in May as part of Gallup Daily tracking. Though the changes in party affiliation are small on an absolute basis, they are meaningful because of the large number of interviews in each month's sample.
That's an enormous sample size, one which yields a margin of error of just 0.6%. Gallup achieves such a massive sample by aggregating an entire month's worth of polls; a normal poll probably has a sample of about 1,000. So while you lose the immediacy of a poll taken over a short period of time, you gain a depth that would otherwise be impossible to obtain.
Gallup further explains the phenomenon we're seeing here:
The recent increase in the Democrats' advantage in party affiliation coincides with Americans' more positive evaluations of President Obama, who averaged 50% job approval in May, compared with 44% in April.
Historical Gallup trends indicate party affiliation often shifts when presidents are very popular or unpopular. For example, Republicans gained an advantage in 1991 after George H.W. Bush's approval ratings soared during and after the Persian Gulf War. Democrats expanded their advantage in party affiliation in the late 1990s during the latter part of Bill Clinton's presidency, when the economy was booming and Clinton's approval ratings routinely topped 60%. Between 2005 and 2008, the Democrats built up a large advantage as George W. Bush's approval ratings suffered due to the Iraq war and later to record-high gas prices and a poor economy.
The current data show a similar pattern, with party affiliation moving toward the president's party as he has gained popularity. However, the magnitude of the change reflects the more modest increase in Obama's approval rating.
Gallup measured a six-point bounce in Obama's approval rating in the days immediately after Osama bin Laden's death, which is typical in size for a presidential rally.
The big question, of course, is whether or not Democrats can maintain this partisan ID advantage, and that hingest heavily on Obama's approval ratings. Some polls (including those taken for Daily Kos & SEIU by PPP) have shown those numbers dipping back down lately, but it's far too early to draw any conclusions about what this might mean for Democratic prospects next year. We'll just have to wait and see.