Editor's note: Julian E. Zelizer is a professor of history and public affairs at Princeton University. He is the author of "Jimmy Carter," published by Times Books, and editor of a book assessing former President George W. Bush's administration, published by Princeton University Press.
Princeton, New Jersey (CNN) -- Republicans are looking ahead -- some with glee and others with fear -- to the presidential election in 2012.
Many in the GOP, ranging from former Speaker of the House Newt Gingrich to Indiana Gov. Mitch Daniels to Donald Trump, have been courting the media to discuss the possibility of running.
New Jersey Gov. Chris Christie, voicing confidence about his own prospects, told reporters that many people had been calling him to say, "Let me explain to you how you could win.' And I'm like, 'You're barking up the wrong tree. I already know I could win.' That's not the issue."
But Republicans have also expressed a significant amount of hesitation about jumping into the race. Christie still says he is not planning to run. Gingrich has been going back and forth. Former Arkansas Gov. Mike Huckabee has delayed a decision and Sen. John Thune of South Dakota has already dropped out.
Most of them, even the optimists, realize that there is a rough road ahead. With all the challenges that President Obama confronts, he won't be easy to beat.
The fact is that it has been extremely difficult in recent history to defeat presidential incumbents who decide to run for a second term. Since Herbert Hoover lost to Franklin Roosevelt in 1932, only three incumbents have been defeated in their re-election bid -- Gerald Ford in 1976, Jimmy Carter in 1980 and George H.W. Bush in 1992. Harry Truman and Lyndon Johnson announced they would not run in the middle of the primaries.
The odds are not great for the GOP given that there is very little chance that Obama won't be at the top of the Democratic ticket.
Several factors seem necessary to prevent a president from being re-elected. The first is a strong primary challenge that weakens support for the president within his own party. In Ford's case, Ronald Reagan mounted an unexpectedly strong campaign by attacking the president for staying too close to the center of the political spectrum.
In 1980, Sen. Edward Kennedy did the same to Jimmy Carter, charging that the president had embraced Republican ideas such as deficit reduction and that he sold out the liberal base of the party. President George H.W. Bush struggled against Patrick Buchanan, who rallied the right wing in the primaries. Before Truman and LBJ dropped out, both of them faced primary challenges.
In 2012, a strong primary challenge to Obama is unlikely. Most prominent Democrats have refrained from even exploring this possibility, despite discontent on the left. If there is a challenge, it is likely to come from someone like Rep. Dennis Kucinich of Ohio, who does not have the stature to cause significant problems for the president.
The second factor in the defeat of incumbents is a bad economy. Here Obama has reason to worry.
Both Gerald Ford and Jimmy Carter governed in an era of stagflation. The nation suffered from unemployment and inflation at the same time (which economists had said was impossible). In Carter's case, the nation had also been shocked by a second oil embargo by OPEC in 1979.
When George H.W. Bush set off to run in 1992, a recession was making Americans worry about their finances and job security. In all three cases, their opponents were able to position themselves as candidates who would revive the economy.
This time around, the economy will probably still be in bad shape. That said, with an improving set of statistics on unemployment and a rising stock market, there is evidence that the economy is moving in the right direction.
If this continues, even if conditions are far from perfect, many Americans will be left with the impression that the economy is on the mend. Obama will doubtless receive some credit for his policies. This will make it more difficult for the GOP, especially since the downturn began under George W. Bush.
The third factor is a foreign policy challenge that undermines perceptions of presidential leadership.
Gerald Ford's opponents charged that the administration was standing still, as a result of the policy of détente, in the face of Soviet expansionism in areas such as Africa. The Iran Hostage Crisis in 1980 made Jimmy Carter appear like a president who had lost control of conditions overseas.
George H.W. Bush, who had enjoyed a dramatic victory against Saddam Hussein in Operation Desert Storm in 1991, had come under serious political attack for not having completed the job. Hussein quickly re-established his power and launched attacks on his opponents that humiliated the president.
Could the Middle East upheaval we're seeing now be a similar challenge to Obama's leadership? Perhaps. But the politics of this region are so messy and there is such division within both parties over what to do that it may be hard for any Republican to put together a coherent case against the policy of the White House.
Finally, defeating an incumbent president requires an exceptional candidate. Jimmy Carter, though later remembered for his failures, was able to put together a very skillful campaign in 1976 that played perfectly to the frustrations Americans had with government in the post-Watergate period.
Ronald Reagan had significant political experience as governor of California and his media skills as an actor made him a powerful force on television and in the radio. More importantly, Reagan was deeply imbedded in the conservative movement, which came out to support him in the campaign.
Although he was not well known at first, Bill Clinton had demonstrated his political acumen as governor of Arkansas, rebounding from a difficult defeat and reinventing his image. He also had been a key figure in the Democratic Leadership Council, which devised and promoted a set of ideas aimed to push Democrats toward the center and to re-energize a tired party. Clinton revealed an uncanny ability to handle the ebbs and flows of campaign politics, connecting with his audiences and offering rapid responses to almost every attack.
Republicans don't have a similar candidate lined up for 2012. The current crop of Republicans includes state officials relatively untested in national political waters, candidates with more experience but lacking charisma, and national veterans like former Massachusetts Gov. Mitt Romney and former Alaska Gov. Sarah Palin with substantial political liabilities. The next Ronald Reagan has yet to arrive.
History should give Republicans some pause as they start to think about their choice. None of this means Obama is a lock for re-election. After all, conditions can change and new voices can rapidly emerge in today's media. But it's going to take a special candidate and a change in the political environment for a Republican to win.
The opinions expressed in this commentary are solely those of Julian E. Zelizer.