Julian E. Zelizer is a professor of history and public affairs at Princeton University. He is the author of "Jimmy Carter" (Times Books) and editor of a book assessing former President George W. Bush's administration, published by Princeton University Press.

...In some ways, Obama is looking back to President Harry Truman's 1948 campaign against Republican Thomas Dewey. In a stunning come-from-behind campaign, where most of the pollsters were wrong in their predictions, Truman railed against the "Do-Nothing Congress" and appealed to core Democratic constituencies such as organized labor to win re-election.

But this style of campaign poses many risks to the president in 2012. Obama does not have the same kind of huge foreign policy record that helped Truman neutralize the GOP in 1948, a campaign that came one year after the Truman Doctrine and the same year as the Marshall Plan....

A populist, anti-Washington campaign is at odds with the Obama whom the nation has seen up close since January 2009. While the president clearly shares many concerns with the left wing of the party, it's also clear that he is much more comfortable staying close to the center. He governs much more like President Bill Clinton than one might have expected from his 2008 primary campaign. Obama is a pragmatist and a politician, less concerned about ideological purity than about political outcomes.

The president's relationship with the left has been extremely tense. During a series of legislative battles, including the one over health care reform, Obama kept his distance from the base of his party in an effort to find legislation that could win over the handful of centrists in his own party and the GOP. Some members of his administration, including former press secretary Robert Gibbs, have made tough statements about the need for liberals to be more realistic about their goals.

Unlike Truman, who had his own problems with the left, Obama cannot point to a huge record from his party of successful policies that have bolstered the security of America's middle class. When Truman ran in 1948, FDR and the New Deal were still fresh on the minds of most voters....