The U.S. Supreme Court handed President Barack Obama a major victory Thursday, upholding the key part of his controversial health care plan in a 5-4 decision.

The so-called “individual mandate” requires Americans to purchase health insurance or face a financial penalty. Chief Justice John Roberts sided with the court's more liberal justices, writing in his majority opinion that “because the Constitution permits such a tax, it is not [the court's] role to forbid it, or to pass upon its wisdom or fairness.''

Justices Stephen Breyer, Ruth Bader Ginsburg, Elena Kagan and Sonia Sotomayor voted with Roberts to uphold the law.

The Obama administration had argued the “individual mandate” is valid under the U.S. Constitution because all Americans will need medical care at some point in their lives, and there is no practical alternative to insurance.

Twenty-six states filed suit against the reform law, contending that individuals cannot be forced to buy insurance, a product they may neither want nor need.

The law has also become one of the most hotly contested issues in the 2012 presidential race. Expected Republican presidential candidate Mitt Romney has vowed to repeal the health care law if he is elected in November.

U.S. Senate Republican leader Mitch McConnell Thursday criticized the Supreme Court ruling and the health-care plan. He said the law the Obama administration pushed through Congress “made the problems it was meant to solve even worse – the supposed cure has proven to be worse than the disease.”

More stinging criticism came from other lawmakers, but the president's supporters applauded the ruling.

The top Democrat in the House of Representatives, Nancy Pelosi, said the court ruling is a victory for the American people because it “ensured health care would be a right for all, not a privilege for the few.”

The U.S. business community is also speaking out.

U.S. Chamber of Commerce President Thomas Donohue said his pro-business group still feels the health-care law is fundamentally flawed. If it is left unchanged, he said, “it will cost many Americans their employer-based health insurance, undermine job creation, and raise health-care costs for all.”

The health care law passed in early 2010 is the signature piece of President Obama's legislative agenda during his time in office. It is intended to address rising health-care costs and the financial problems many Americans have faced in trying to obtain health insurance.