(PhysOrg.com) -- President Obama's campaign brought millions of new voters to polls during the 2008 elections, but the decisions of former Bush voters had a substantial effect on the outcome.
A new University of Michigan analysis indicates that several million formerly Republican voters chose not to support party nominee John McCain, either staying home during the elections or opting for Barack Obama.
If new Obama voters are less likely to go to the polls in 2012, a critical swing vote could be voters who chose Bush in 2004.
"Future presidential hopefuls' attempts to draw lessons from the 2008 campaign should focus not only on how the Obama campaign got so many new people to the polls," U-M political science professor Arthur Lupia said, "but also on why so many people who voted for a Republican presidential candidate in 2004 chose to do something different in 2008."
More than 69 million Americans cast votes for Obama in the November election. This number is 10 million more than people who voted for fellow Democrat John Kerry four years earlier.
"Obama's success in attracting new voters was a visible component of his 2008 victory … but these new voters were not a necessary condition for his victory," said Lupia, a research professor at the Institute for Social Research.
Secret ballot prevents people from knowing the exact number of Bush 2004 voters who failed to vote for McCain in 2008. However, Lupia used state-level election returns to determine the minimum number of such voters.
For example, a minimum of seven out of every hundred Bush voters in Ohio in 2004 must have done something other than vote for John McCain in 2008. Winning this state was sufficient for Obama to win the Electoral College, Lupia noted.
"Therefore, Bush voters' decisions in Ohio were sufficient to cause Obama to win the election," he said.
Other survey data reinforces the role that Bush voters played in Obama's victory. The 2008 edition of the American National Election Studies Time Series (American National Election Studies 2008)—a project involving U-M and Stanford University researchers—asked more than 2,000 people for whom they voted in the general elections of 2004 and 2008.
Nearly one in four respondents who self-identified as 2004 Bush voters did something other than vote for McCain in 2008. Fifteen percent of these Bush voters voted for Obama in 2008. Another 7 percent chose not to vote at all, the ANES data showed.
Lupia, the Hal R.Varian Collegiate Professor of Political Science, conducts research on how voters, political elites, and scientific experts make decisions when they lack important information about consequences
The findings appear in PS: Political Science and Politics.
Explore further: Long-term survey to follow college students' experiences with faith, diversity
More information: PS: Political Science and Politics - www.apsanet.org/PS/