Polls and Elections: Preelection Poll Accuracy in the 2008 General Elections

Voters in the 2008 presidential election decidedly endorsed Democrat Barack Obama over his Republican opponent John McCain in the first-ever contest between two sitting U.S. senators. Amid growing economic turmoil and frustration with the war in Iraq and with two-term incumbent Republican president George W. Bush, Obama bested McCain at the polls by nearly 10 million votes. The historic election attracted 61.6% of the eligible electorate to the polls on election day--the highest turnout since 1968--and produced the first African American president of the United States (McDonald 2009). Obama ultimately captured 52.9% of the popular vote, while McCain received 45.7%. The Democratic nominee carried 28 states plus the District of Columbia and prevailed in Nebraska's Second Congressional District to garner 365 electoral votes; McCain earned 173 electoral votes from victories in 22 states. (1) The 2008 election was the first election in 56 years that did not include an incumbent president or vice president. Both parties held spirited primaries that attracted a broad range of candidates, but the Democratic primary race that pitted former first lady and U.S. senator Hillary Clinton, the early front-runner, against Barack Obama was especially vivacious. The 17-month contest remained close throughout, and Obama did not secure enough delegates to clinch the nomination until June 3, 2008. Despite noteworthy preelection polling setbacks during the primaries (most pollsters predicted an Obama victory in New Hampshire, for example, but Clinton won) (Liss 2008), the final national preelection polls in the general election unanimously projected a Democratic victory. Estimates of Obama's margin over McCain were among the most precise on record. In this essay, I analyze national and subnational preelection polls conducted in the 2008 cycle to present an in-depth evaluation of poll accuracy. Preelection Poll Volume and Poll Aggregation in 2008 The number of preelection polls conducted during a typical presidential election cycle has grown substantially over the past few decades. Michael Traugott reports that the "explosion" in polls started in the 1980s and that the occurrence of the standard trial heat items in polls increased about 900% between 1984 and 2000 (2005, 644). The overall volume of preelection polling conducted during the 2008 cycle remained high. Based on national samples reported on Pollster.com, 221 non-overlapping presidential trial heats were conducted between Labor Day (September 1) and election day 2008, substantially more than in 2004 (although fewer than in the 2000 cycle). (2) Several polling organizations conducted daily tracking polls in 2008; Gallup and Rasmussen tracked preferences daily for at least six months leading up to the November election. In addition, DailyKos, George Washington University Battleground, Hotline, and Zogby provided daily tracking estimates during the fall campaign, and two additional organizations, IBD/TIPP and ABC News/Washington Post, reported daily tracking poll data toward the end of the campaign. A total of 975 trial heat items in statewide polls fielded between Labor Day and election day probed presidential preferences in 2008, while 394 trial heats reported statewide preferences for U.S. Senate candidates and 110 for gubernatorial candidates over the same duration. One of the main developments in terms of preelection polling in 2008 was the emergence of online poll aggregators. Web sites such as FiveThirtyEight.com, Pollster.com, the Princeton Election Consortium, and RealClearPolitics.com collected, analyzed, and graphed an unprecedented amount of state and national polling data and helped voters interpret and assess campaign dynamics by providing singular, statistical measures that captured all of the available information daily. Poll aggregators used different mathematical models to contextualize polling information and to refine the predictive capabilities of available polling data (Harmanci 2008). …