PRINCETON, NJ -- In May, 45% of Americans identified as Democrats or said they were independent but leaned Democratic, compared with 39% who identified as Republicans or leaned Republican. The six-percentage-point Democratic advantage represents a slight increase from the four-point advantage Gallup measured in April, which matches the 2011 average to date.
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.
Longer term, the current six-point Democratic edge is the largest measured in Gallup Daily tracking since October 2009, when the gap was seven points. The high point in the more than three-year history of Gallup Daily tracking is 19 points in December 2008. Since President Obama took office, that gap has generally shrunk over time to the point that the parties have been more competitive in recent months, including a tie in August 2010 tracking.
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.
Had the rally been larger, party affiliation likely would have shifted even more in a Democratic direction. For example, between the third and fourth quarters of 2001, spanning the 9/11 terrorist attacks, the party affiliation gap shifted six points toward the Republicans as George W. Bush received record-high approval ratings.
Whether the Democrats are able to maintain their now slightly larger advantage will depend partly on whether Obama's approval rating continues to register at or near the improved 50% level.
Results are based on telephone interviews conducted as part of Gallup Daily tracking May 1-31, 2011, with a random sample of 30,239 adults, aged 18 and older, living in all 50 U.S. states and the District of Columbia.
For results based on the total sample of national adults, one can say with 95% confidence that the maximum margin of sampling error is ±1 percentage point.
Interviews are conducted with respondents on landline telephones and cellular phones, with interviews conducted in Spanish for respondents who are primarily Spanish-speaking. Each sample includes a minimum quota of 400 cell phone respondents and 600 landline respondents per 1,000 national adults, with additional minimum quotas among landline respondents for gender within region. Landline telephone numbers are chosen at random among listed telephone numbers. Cell phone numbers are selected using random-digit-dial methods. Landline respondents are chosen at random within each household on the basis of which member had the most recent birthday.
Samples are weighted by gender, age, race, Hispanic ethnicity, education, region, adults in the household, and phone status (cell phone only/landline only/both, cell phone mostly, and having an unlisted landline number). Demographic weighting targets are based on the March 2010 Current Population Survey figures for the aged 18 and older non-institutionalized population living in U.S. telephone households. All reported margins of sampling error include the computed design effects for weighting and sample design.
In addition to sampling error, question wording and practical difficulties in conducting surveys can introduce error or bias into the findings of public opinion polls.
For more details on Gallup's polling methodology, visit www.gallup.com.