Editor's note: Julian E. Zelizer is a professor of history and public affairs at Princeton University's Woodrow Wilson School. His new book, "Arsenal of Democracy: The Politics of National Security -- From World War II to the War on Terrorism," will be published this fall by Basic Books. Zelizer writes widely about current events.
Julian Zelizer says Obama's biggest challenge is the split within his own party in Congress.
PRINCETON, New Jersey (CNN) -- The second hundred days of Barack Obama's presidency have in many ways been more revealing than the first hundred days.
We have learned a lot, not so much about Obama's governing style or his policy agenda, but about the political environment in which he will have to operate -- at least until the 2010 midterm elections.
Obama has experienced some difficult challenges since April. On paper, the administration has clearly not achieved the same kind of sweeping early legislative record as his Democratic predecessors such as Franklin Roosevelt or Lyndon Johnson. We have not witnessed the creation of another New Deal or Great Society.
The economic stimulus bill is the most ambitious legislation to move through Congress thus far. The financial and economic bailout program has proved extremely important as well, but that comes from the Troubled Asset Relief Program begun by President George W. Bush at the height of the financial crisis.
The two big-ticket items for the administration -- health care and the environment -- are stalled in Congress. There is a potential compromise looming on health care that would fall far short of Obama's campaign promises. At the same time, most of the national security policies that were put into place after September 11 by the Bush administration remain intact.
Many of the challenges confronting Obama have resulted from the difficulties of the environment in which he governs.
One of the biggest challenges has been the division within the Democratic Party between a handful of centrists and the liberal base. The tensions immediately became apparent when moderates forced Congress to reduce the size of the economic stimulus bill back in February.
That was just a taste of things to come. The intra-party divisions have become more pronounced in the debate over health care. Sens. Max Baucus and Kent Conrad have held their ground by resisting the public insurance option endorsed by the White House as well as many of the finance proposals that have been made in Congress.
House Speaker Nancy Pelosi has been forced to contend with a vocal group of Blue Dog Democrats who want to impose more stringent cost-cutting measures and are also resisting paying for benefits with a surtax on high-income earners.
There is little evidence that these tensions will dissipate, and they exist with other issues as well, including environmental legislation. As a result, Democrats will lose some of the institutional benefits that they might have expected from having a 60-person majority in the Senate.
The second problem for Obama is that partisan polarization is alive and well. Despite all the talk about bipartisanship in the 2008 campaign, the sources of polarization are deeply rooted in our modern political system. Therefore, even with a new president in town, the same kinds of partisan wars have been playing out.
There is little love on either side of the aisle. Republicans have been relatively consistent in lining up against the administration's proposals. The Senate vote for Judge Sonia Sotomayor's confirmation to the Supreme Court is expected to draw fewer Republican votes than the number of Democrats who voted for George W. Bush's nominees, John Roberts and Samuel Alito.
With Obama's approval numbers dropping, Republicans will certainly come back from their summer break even more confident about saying no to him. While the improvement of the economy could give the president a boost, it will also diminish the sense of crisis that had put Republicans on the defensive.
A third challenge is that the the government reform efforts that Obama touted in his campaign haven't materialized, and thus Washington still works the way it did before he took office. As Americans have seen so many times, promises about government reform quickly fade once a campaign ends.
Although Obama did implement some new lobbying rules upon taking power, he has shown minimal interest in the procedural problems that he highlighted on the campaign trail: the power of interest groups, the role of private money in campaigns, the revolving door between K Street and Capitol Hill and more.
Back in January 2006, he told a lobbying summit at the National Press Club that, "The American people are tired of a Washington that's only open to those with the most cash and the right connections. They're tired of a political process where the vote you cast isn't as important as the favors you can do. And they're tired of trusting us with their tax dollars when they see them spent on frivolous pet projects and corporate giveaways."
His point, one that he made many times, was that without reforming the political process, it is virtually impossible to change policy and restore trust in government. But the process has not changed, and there is no evidence that it will. As a result, Obama will be attempting to obtain new policies in a town where interest groups and lobbyists remain organized, powerful and committed to the status quo.
Finally, Obama has learned how difficult it is to retrench public policies. Presidents often have to live with the world they inherit. Conservatives learned this lesson with domestic policy. In 1981 and 1982, Ronald Reagan was forced to back down from proposed cuts to Social Security after encountering a political backlash. George W. Bush discovered the same thing in 2005.
Although Obama's campaign started with a promise of fundamentally changing the direction of national security policy, in the second hundred days, there has not been much evidence that he will be able to accomplish those goals, outside of completing the scheduled withdrawal from Iraq (though leaving more troops there than many hoped for).
As Jack Goldsmith, a former Bush administration official and a leading public critic of Bush, documented in a recent article in The New Republic, Obama has kept most of President Bush's post-9/11 national security programs in place while resisting efforts to conduct an investigation into potential abuses during the Bush years.
Goldsmith wrote, "the new administration has copied most of the Bush program, has expanded some of it, and has narrowed only a bit." While troops are leaving Iraq, the United States is enlarging its troop presence in Afghanistan.
Obama's first challenge in the third hundred days must be to diminish problem number one. Until he can unite Democrats more effectively, it will be almost impossible to achieve major legislative victories.
Without major victories, his opponents won't be scared about taking him on. If this is the case, the third hundred days might look much less like the promise of the 2008 election and more like the political world that Obama wanted to transform.
The opinions expressed in this commentary are solely those of Julian Zelizer.