The Supreme Court has upheld the centerpiece of President Obama’s health care overhaul, in effect allowing the law to survive. 

In a 5-4 decision unveiled Thursday, the court ruled as constitutional the so-called individual mandate requiring most Americans to obtain health insurance starting in 2014. 

The ruling is a victory for the president, ensuring for now that his signature domestic policy achievement remains mostly intact. 

It also ensures that the law will play a prominent role in the general election campaign, as Republican candidate Mitt Romney vows to repeal the law if elected.

Chief Justice John Roberts, who was appointed during a Republican administration, joined the four left-leaning justices on the bench in crafting the majority decision. 

The ruling relied on a technical explanation of how the individual mandate could be categorized. Roberts, in the opinion, said the mandate could not be upheld under the Constitution’s Commerce Clause. However, it could be upheld under the government’s power to tax. 

“The Affordable Care Act is constitutional in part and unconstitutional in part  The individual mandate cannot be upheld as an exercise of Congress’s power under the Commerce Clause,” Roberts wrote. “That Clause authorizes Congress to regulate interstate commerce, not to order individuals to engage it. In this case, however, it is reasonable to construe what Congress has done as increasing taxes on those who have a certain amount of income, but choose to go without health insurance. Such legislation is within Congress’s power to tax.”

The ruling did rein in one element of the law — the expansion of Medicaid across the country to take in millions of low-income Americans. The opinion allows Washington to offer more funding to states to expand the program, but says the federal government cannot penalize states for not participating in the new program by withholding existing Medicaid funds.

Justice Anthony Kennedy, who was thought to be the swing vote on the decision, joined the minority in describing the entire law as invalid. 

“The act is invalid in its entirety,” Kennedy said from the bench. He went on to say the administration went to “great lengths to structure the mandate as a penalty, not a tax.”