Julian Zelizer is a professor of history and public affairs at Princeton University. He is the author of "Jimmy Carter" and "Governing America."

(CNN) -- The politics of health care is changing fast. President Barack Obama's Affordable Health Care Act was vulnerable during his first term when Republicans demanded repeal of the law. Even after the Supreme Court upheld its constitutionality, there were still many voices who objected to it.

However, with each passing day, it appears that the program is in good shape, slowly becoming part of the fabric of American government.

Last week, New Jersey Gov. Chris Christie, one of the main potential contenders for the Republican presidential nomination in 2012, said that his state would accept the Medicaid expansion that is part of the ACA. Christie had been one of the president's toughest critics, frequently lambasting the program as a prime example of big government liberalism. But he has changed his tune.

The expansion of Medicaid will allow about 104,000 of the poorest residents in New Jersey to gain access to health insurance. Christie said: "Let me be clear: I am no fan of the Affordable Health Care Act. I think it is wrong for New Jersey and for America. I fought against it and believe, in the long run, it will not achieve what it promises. However, it is now the law of the land. I will make all my judgments as governor based on what is best for New Jerseyans."...