Editor's note: Julian E. Zelizer is a professor of history and public affairs at Princeton University's Woodrow Wilson School. His new book is "Arsenal of Democracy: The Politics of National Security -- From World War II to the War on Terrorism" published by Basic Books. Zelizer writes widely about current events.
Princeton, New Jersey (CNN) -- In the week since Republican Scott Brown's victory in Massachusetts, President Obama has shown signs that he is preparing to move further to America's political center.
In many respects, Obama has reached a fork in the road and must decide whether he plans to veer to the center at the expense of his increasingly discontented liberal base.
We won't know the full outlines of the president's new approach until his State of the Union address on Wednesday. But there are strong indications: He will propose a three-year freeze on discretionary spending in most government agencies and, over the weekend, he endorsed the idea of creating a commission to make recommendations for reducing the budget deficit.
While not quite as dramatic as Bill Clinton's announcement in his 1996 State of the Union address that the "era of big government is over," Obama is signaling that he wants to appeal to centrist voters concerned about government spending.
On the other hand, at the end of last week, President Obama made some populist appeals as he called for stricter rules and regulations for the banks.
The real test will come with what he does after the speech. The conventional wisdom suggests that Obama should probably go for the center. This is the safe bet, particularly for a Democrat in a defensive position.
Indeed, the president will still need to win over some moderate Democrats -- and now Republicans -- if he wants to avoid a filibuster that could block his legislative agenda. He also does not want to alienate moderate Democrats and independent voters who could swing Republican if he is perceived as being too far to the left.
But Obama has already bargained with moderate Democrats throughout his first year and the results have not been great. He should be cautious about jumping too far to the middle because that, too, could have its costs.
There are many liberals who are deeply unhappy with the president, believing that he has already drifted too far away from the promises that animated his supporters in 2008. Obama will need liberals to mobilize the vote in the 2010 and 2012 elections. He needs liberals to build public support for legislation that thus far has stalled.
President Obama also cannot afford an open battle with liberals. It could leave him without any strong base of support and fuel the perception that he is an ineffective leader, something else independent voters don't tend to appreciate.
There is a long tradition of Democratic presidents taking the left for granted at a cost to their administrations. These presidents learn that the ire of the left -- a constituency that is very vocal, highly mobilized and politically engaged -- can cause enormous damage.
President Johnson famously dismissed early warnings that liberals were unhappy with the war in Vietnam. When he first heard about college sit-ins that were taking place against Vietnam in 1965, the president told Undersecretary of State George Ball: "Don't pay attention to what those little ---- on the campuses do. The great beast is the reactionary element in the country."
Yet the anti-war movement emerged as a powerful force in domestic politics, receiving support from mainstream Democrats like Sens. Robert Kennedy and Eugene McCarthy. When Democrats were torn apart by a civil war during the 1968 Democratic Convention in Chicago, Illinois, Johnson realized that he had underestimated the costs of his choice.
President Carter also disappointed liberals after his first year in office. The president decided to focus on controlling inflation and lowering deficits. He put aside his promise to achieve national health care reform and to lower unemployment. Liberal groups expressed their frustration. Massachusetts Sen. Ted Kennedy decided to challenge Carter in the 1980 Democratic primaries.
Castigating the administration, Kennedy said that the Democratic Party needed to "sail against the wind" of conservative public sentiment by using the federal government to help alleviate social problems. While Carter won, the battle between the two left the party much weaker going into the general election. Ronald Reagan took advantage.
President Clinton constantly disappointed liberals as well. During his first year in office, Clinton pushed NAFTA through Congress and angered Midwestern Democrats who feared the economic consequences of free trade. He settled on the "don't ask, don't tell" compromise over homosexuals in the military, which belied his promise to change policy. After Republicans took control of Congress in 1994, Clinton announced that he would govern from the center.
His move toward the center did help him win re-election, but it cost him allies on legislation. The tensions between Clinton and liberals never disappeared even though many Democrats rallied to his side during the impeachment trial.
With Republicans unwilling to help him win votes and Democrats not particularly interested in supporting him, Clinton's legislative record on domestic policy ended up being rather thin. In certain respects, Sen. Hillary Clinton paid the price of the tensions as many liberal Democrats chose Obama in 2008, remembering the Clinton era in the White House.
Today, President Obama, just one year into his presidency, finds himself facing the same difficult choice.
If he wants to hold onto enthusiastic liberal support, the president must argue passionately and energetically for jobs programs and finishing work on health care reform. Unlike Clinton after 1994, Obama still has a Democratic Congress, with sizable majorities.
Liberals can help Obama to stimulate support for his legislative proposals, increasing pressure on moderate Democrats, and to develop a reputation for being an effective and action-oriented leader -- a reputation that can be as compelling to moderates as evidence of trying to cut deficits.
If the president backs too far away from the issues that animated his supporters in 2008, he could find himself facing even stronger challenges from liberals and depressing the base of support that he will very much need going into 2012.
The opinions expressed in this commentary are solely those of Julian Zelizer.