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....