Updated: 11:18 a.m.

The Supreme Court today upheld the 2010 health care law, validating President Barack Obama’s top domestic policy achievement while dealing a stinging rebuke to mostly Republican critics who charged the statute was unconstitutional.

The opinion gives legal certainty to one of the most sweeping pieces of domestic policy legislation since President Lyndon B. Johnson’s Great Society programs in the 1960s and allows insurers, hospitals, drugmakers and other health care providers to continue planning for sweeping changes, many of which are set to take place in 2014.

Twenty-six states and the National Federation of Independent Business challenged a provision in the law requiring most Americans to buy health insurance or pay fines, saying it exceeded legislators’ power over interstate commerce. States also challenged an expansion of Medicaid, a health care program for the poor.

Chief Justice John Roberts joined four Democratic-appointed justices in ruling 5-4 that the coverage mandate was legal under Congress’ authority to levy taxes. The court limited, but did not invalidate the Medicaid  expansion, saying the federal government  can’t withhold funds from states that don’t fully comply with the provision. Justices Antonin Scalia, Anthony Kennedy, Clarence Thomas and Samuel Alito dissented.

Other elements of the law remain intact, including a requirement that health care plans offer coverage to individuals with pre-existing conditions and provisions that allow young adults to stay on their parents’ insurance until age 26 and help close a gap in seniors’ Medicare drug coverage.

The dramatic ruling, coming a little more than 18 weeks before the presidential election, probably won’t end the bitter partisan rancor surrounding the measure, known as the Affordable Care Act.

The Democratic-controlled House and Senate passed the law without a single Republican vote in March 2010, and polls have shown the public almost evenly divided over its merits ever since.

Senate Minority Leader Mitch McConnell (R-Ky.) said the high court ruling underscores the need for Congress to repeal the entire law.

“Today’s decision does nothing to diminish the fact that Obamacare’s mandates, tax hikes and Medicare cuts should be repealed and replaced with common sense reforms that lower costs and that the American people actually want,” McConnell said.

Senate Majority Leader Harry Reid (D-Nev.) said millions of Americans are already seeing benefits from the law and called on lawmakers to move on to address the economy.

“Now that this matter is settled, I hope we can work together to create jobs and secure this country’s economic future,” Reid said.

In NBC News/Wall Street Journal poll of 1,000 adults conducted June 20-24, 37 percent said they would be pleased if the law was overturned by the court, while 22 percent said they would be disappointed with that outcome. In contrast, 28 percent said they would be pleased if the justices found the law constitutional, while 35 percent said they would be disappointed with such a ruling.

Overall, 35 percent of respondents think the health care overhaul is a good idea, compared with 41 percent who believe it’s a bad idea. The split has been more or less the same since the law was enacted.

The law was designed to reshape the medical system by expanding coverage to as many as 32 million uninsured people. It calls for the establishment in 2014 of regulated marketplaces called exchanges, where individuals and small businesses could buy coverage. It also provides tax credits for part of the cost of premiums for low-income households.

The statute establishes new advisory bodies to help rein in health care spending. One, the Independent Payment Advisory Board, was charged with drafting legislative proposals to slow the rise in Medicare spending but became a lightning rod for critics, who said it was a back-door attempt to ration care and limit patient choice.

The law also contains revenue-raisers, including an excise tax on high-cost insurance plans offered by employers, a new 2.3 percent tax on medical devices and fees on drug companies and health insurers.

The costs of implementing the law kept the health care debate alive long after the measure was enacted. Earlier this year, Congressional Republicans questioned why White House spending estimates for subsidies for low-income and working-class families would total $478 billion through 2021, $111 billion higher than a year earlier.

The administration said most of the difference was due to changes Congress made to the law since it was signed.