Editor's note: Julian Zelizer is a professor of history and public affairs at Princeton University. He is the author of "Jimmy Carter" and of the new book "Governing America."
(CNN) -- White House officials rightly breathed a sigh of relief when the Supreme Court upheld the Affordable Care Act. President Obama and his supporters are optimistic that his re-election prospects are stronger and his lasting impact on domestic policy will be much greater.
But the battle has only begun. Even after legislation is enacted, as the political scientist Eric Patashnik says in his book "Reforms at Risk," its survival is not assured. The key to success is whether a program lasts over time and builds strong political support to protect it from future attacks. As Patashnik and political scientist Jeffrey Jenkins wrote in a recent blog post, before the Supreme Court ruled: "The partisan and ideological struggle over health-care reform is likely to continue under any scenario."
Congressional Republicans will likely continue attacking the program's funding. House Speaker John Boehner has vowed to push for its repeal.
Mitt Romney has said he will do the same if he is elected president.
Congressional Republicans will use the budgetary axe to make sure that ACA does not have a smooth path to success. The Supreme Court's decision on Medicaid has opened up questions about whether red state governors will follow through with an essential expansion of the program.
There are also challenges in making the law work. Who will enforce penalties on those who refuse to buy insurance? How effective will the cost controls be? States setting up their health care exchanges will need to take tremendous care so that consumers really receive the best choices about where to purchase their insurance. Some state officials, in Idaho and South Dakota for example, are refusing to implement the law until after November. Will the federal government step in to set up these states' health care exchanges?
The good news for Obama is that many sturdy, well-regarded federal programs did not start out that way. Social Security did not do well at first. A freeze on taxes during World War II jeopardized the program, and there were real concerns that it would not survive, given that other programs for the elderly were more popular.
But Congress made adjustments, expanding coverage to include groups such as widows, increasing benefits and revising the tax system to a pay-as-you-go basis, with monthly benefits funded by monthly taxes. By 1950, the program was sound and received bipartisan support.
When the Supreme Court struck down an agricultural-processing tax that supported a farm subsidy program in 1936, Congress reworked it, resolving the constitutional problems and protecting farmers.
Other periods have seen similar adjustments. Congress passed a watered-down voting rights bill in 1957, one that frustrated liberals.
Lyndon Johnson, then the Senate majority leader, promised it would be improved upon and expanded in future years.
"It's just a beginning," he told liberals. "We've shown that we can do it, we'll do it again."
He was right. In 1964 and 1965, Congress passed far bolder legislation.
Medicare, which Congress enacted in 1965, has faced the chronic challenge of cost control. In the 1980s, Congress imposed regulations on what hospitals could charge the program to help contain rising costs.
Congress has taken similar action with taxes. When President Ronald Reagan's 1981 tax cut and increased military spending made the deficit balloon, Congress -- with the support of key Republicans like Sen. Robert Dole of Kansas -- raised taxes in 1982 and 1983.
Congress' job is not just to make new laws, but also to fix existing ones. The question now is whether Congress can overcome polarization and gridlock to refine and improve the health care law to make sure it works. In many respects, the 2012 election will be a much bigger moment for health care than the Court decision. If Democrats don't make sure this law works well, a future Congress might use its power to repeal the legislation and finish the job they wish the Supreme Court had done.
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The opinions expressed in this commentary are solely those of Julian Zelizer.