Editor's note: Julian Zelizer is a professor of history and public affairs at Princeton University. He is the author of "Jimmy Carter" (Times Books) and of the new book "Governing America" (Princeton University Press).
(CNN) -- Republicans warn that if President Barack Obama wins a second term, he will push forward with an expansive domestic liberal agenda that makes his existing record look like child's play. During the victory speech that followed the Michigan primary, Mitt Romney warned that "a second term Obama would be unrestrained by the demands of re-election."
If you didn't like health care and financial regulation, the Republicans are saying, then you really won't like what happens when things turn leftward after 2012. No longer forced to think about independent voters and suburban moderates, Obama will allow his true liberal values to shine, the theory goes.
But there is little evidence that Obama would move sharply to the left in a second term, regardless of which party controls Congress. Most likely, the president would focus his energy on protecting the programs that Congress enacted in his first term, namely health care, while he would turn his attention to narrower issues, such as specific infrastructure projects, that seem politically viable. In addition, Obama might co-opt some Republican themes by moving forward on Social Security reform and deficit reduction.
Why? Despite the Republican claims about Obama being a big-government liberal, the truth of the matter is that he is a pragmatic centrist to the core.
Repeatedly, Obama has frustrated liberals by forcing them to accept compromises on almost every issue, ranging from the type of health care reform that he proposed to the size of the economic stimulus. He has continued many of President George W. Bush's programs, including TARP and the tax cuts for the wealthiest Americans (although he advocates getting rid of the tax cuts when they expire in 2013). Obama has usually embraced market-based approaches to dealing with domestic problems.
According to Ezra Klein of The Washington Post, Obama has been the "most polarizing moderate ever." Klein cites a scholarly study, which shows that Obama is the most moderate Democrat since World War II.
Equally important, it is rare for second-term Democratic presidents to do more on domestic policy -- in terms of passing big ticket legislation -- than they did in the first. The record of the 20th century shows that presidents narrow their legislative ambitions and, when they don't, they are forced to pull back.
Woodrow Wilson started his presidency with sweeping initiatives in 1913, but his second term was consumed by the demands of World War I.
In the same way, President Franklin Roosevelt came out swinging in his first term as he and the Democratic Congress remade American government. The high mark came in 1935, when Roosevelt signed the Wagner Act that legitimated unions as well as the Social Security Act.
Following a fierce backlash in 1937, Roosevelt scaled back his domestic proposals, after which wartime policies became the focus of his effort as well. Though he pushed through Congress some big changes in government during war, such as price controls, his ability to obtain major domestic programs such as Social Security diminished.
President Harry Truman ramped up his demands after 1948, emboldened by his unexpected victory against Republican Thomas Dewey, but fell back quickly when the conservative coalition of Democrats and Republicans knocked down proposals such as national health care and used anti-communist rhetoric to put Democrats on the defensive.
Lyndon Johnson chose not to run for another term in the 1968 election, though based on the revival of the conservative coalition that took place in 1966 and his growing focus on taxes, inflation and deficits, it is unlikely he would have obtained much more in the way of bold initiatives such as Medicare.
And President Bill Clinton never replicated the effort in 1993 to pass national health insurance. Instead, his second term focused on deficit reduction, Medicare reform, implementing welfare reform and narrower measures such as the creation of the v-chip to protect under-aged television viewers from edgy programs and the expansion of Medicaid to cover children.
Nothing in Obama's record or the history of 20th century Democratic presidents suggests that a second term would be unrestrained. American politics does not allow for such freedom given the role that Congress plays in the political process and the desire of presidents to retain strong approval ratings.
Even if Democrats retook control of both chambers, Senate Republicans would continue to exert enormous influence as a result of the filibuster. And when presidents do big things in their first term, such as health care, they need to make sure that in their second term those programs are protected and set up effectively.
If the president is granted a second term, voters would probably see a leader who is more timid than the first term Obama, someone who might even embrace some Republican issues along the way.
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The opinions expressed in this commentary are solely those of Julian Zelizer.