Editor's note: Julian E. Zelizer is a professor of history and public affairs at Princeton University. He is the author of "Arsenal of Democracy" and a book on former President Carter and editor of a book assessing former President George W. Bush's administration, to be published this fall by Princeton University Press.

Princeton, New Jersey (CNN) -- If current polls are a guide, the midterm elections probably won't be good for President Obama and his party. The Democrats are in danger of losing control of the House of Representatives and of seeing their majority in the Senate diminish.

With Obama's approval rating sagging to 45 percent according to a Reuters-Ipsos poll, even his most ardent supporters admit that he will need a stimulus act for his presidency before 2012 comes around.

One of Obama's biggest challenges has been his reticence about defining a clear agenda and a set of governing principles. Doing so has been at odds with his legislative strategy, which has hinged on avoiding big proclamations to give himself wiggle room with Congress.

But that legislative strategy has had a major political cost. Many Democrats don't feel as if they know exactly what their leader is about. Some conservative critics have been able to paint a rather pragmatic Democrat as a Soviet-style socialist, and sometimes, according to recent polls, as a Muslim.
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To bounce back, Obama will need to do more to articulate his agenda. For guidance, he can look back to two presidents who recovered from difficult midterms, Republican Ronald Reagan and Democrat Bill Clinton.

In 1982, Democrats gained 26 seats in the House. In 1994, Republicans took control of both the House and Senate for the first time since 1954. Both presidents, however, coasted to victory in their bids for re-election.

In both cases, the most obvious factor was economic recovery. For Reagan, the recovery from the 1982 recession made many Americans more confident that his economic policies, namely the tax cuts of 1981, were responsible for ending the economic malaise.

President Reagan was able to run in 1984 on a campaign that promised voters it was "Morning in America Again," with images of Americans going to work and taking care of their homes. Reagan's approval ratings had risen from 35 percent to 58 percent by election time.

President Clinton, who ran against President George H.W. Bush in 1992 by focusing on the economic recession, saw his standing improve as the economy boomed in the second half of the 1990s. The economy revived as the growing technology industry inspired faith in the country.

Republicans and Democrats gave the economy good grades. Unemployment fell to about 5 percent. "Ten million new jobs. Family income up $1,600," boasted the narrator in one of Clinton's 1996 ads. Clinton's approval rating steadily rose in his second term, reaching a high of 73 percent in December 1998 (at the time of the impeachment).

But economic recovery was not enough. In each case, Reagan and Clinton were able to find ways to pursue a strategy of principled compromise that kept the legislative agenda moving forward but simultaneously gave voters a clear sense of what the presidents stood for.

Reagan showed more flexibility on several issues, including his decision to accept tax increases that were being pushed by fiscal conservatives in both parties. He also tempered some of his rhetoric toward the Soviet Union.

At the same time, he never departed from championing the twin themes on which he ran in 1980 -- anti-communism and tax cuts. He spoke about these themes and defended his positions with a passion that could inspire the most frustrated supporters.

When election time came around in 1984 against Democrat Walter Mondale, voters had developed a strong impression of who and what Reagan stood for, even if he had made compromises along the way.

Clinton also enjoyed a recovery after a disastrous midterm election. Clinton made deals with the GOP, focusing on smaller domestic initiatives, working on law and order policies, and signing a bill that ended the public welfare system. Clinton also found his voice once he had Speaker Newt Gingrich as an opponent.

During the heated budget battles that resulted in a government shutdown in 1995-1996, Clinton stood firm as a defender of the key pillars of government support for the middle class, Social Security and Medicare.

In the wake of the Oklahoma City bombing by right-wing extremists in 1995, Clinton aggressively defended the importance of government while warning of extremism on the right. He placed the GOP in a difficult position when House Republicans opposed his counterterrorism initiatives.

As with Reagan and Clinton, Obama's biggest concern in 2011 and 2012 will be whether the economy experiences a more robust rebound. Without this, Republicans will have a much stronger chance of capturing the White House.

But Obama must also do a better job at telling voters what he is about. While the president has a large legislative record to boast of, it remains unclear to many voters, including Democrats who support much of the record, what it all adds up to.

It's time for Obama to state his agenda and lay out a set of governing principles that will guide him in his next two years as president. It's likely that the pressure of the 2010 midterm elections will compel Obama to present an argument to the public to build a case for his presidency.

The opinions expressed in this commentary are solely those of Julian E. Zelizer.