WASHINGTON (AP) — THE RULING:

The Supreme Court upheld the constitutionality of President Barack Obama's health care law, including the most disputed part: the mandate that virtually all Americans have health insurance or pay a fine. The mandate was upheld under the federal government's power to levy taxes.

The ruling put some limits on the law's plan to expand the Medicaid insurance program for the poor, a joint effort of the federal government and states. It says the U.S. government cannot threaten to withhold a state's entire Medicaid allotment if it doesn't participate in the expansion.

Chief Justice John Roberts sided with the court's four liberal justices — Stephen Breyer, Ruth Bader Ginsburg, Elena Kagan and Sonia Sotomayor — to form the 5-4 majority.

THE CONTEXT

The decision affects nearly every American and marks a major milepost in a century of efforts to make health care available to all. The law is President Barack Obama's signature legislative achievement and perhaps the most polarizing issue of his re-election campaign. His Republican rival Mitt Romney and GOP lawmakers have promised to repeal Obamacare.

WHAT NOW?

The 2010 health care law will continue phasing in as planned. It's expected to bring coverage to about 30 million uninsured people, so that more than 9 in 10 eligible Americans will be covered.

Some parts are already in effect: Young adults can stay on their parents' insurance up to age 26. Insurers can't deny coverage to children with health problems. Limits on how much policies will pay out to each person over a lifetime are eliminated. Hundreds of older people already are saving money through improved Medicare prescription benefits. And co-payments for preventive care for all ages have been eliminated.

WHAT'S NEXT?

Starting in 2014, almost everyone will be required to be insured or pay a fine. There are subsidies to help people who can't afford coverage. Most employers will face fines if they don't offer coverage for their workers. Newly created insurance markets will make it easier for individuals and small businesses to buy affordable coverage. And Medicaid will be expanded to cover more low-income people.

Insurers will be prohibited from denying coverage to people with medical problems or charging those people more. They won't be able to charge women more, either. During the transition to 2014, a special program for people with pre-existing health problems helps these people get coverage.

An assortment of tax increases, health industry fees and Medicare cuts will help pay for the changes.

IS THE ISSUE SETTLED NOW?

Not necessarily. Although the court found it constitutional, the health care law still could be changed by Congress. Romney and Republican congressional candidates are campaigning on promises to repeal it if elected in November.

Some parts of the law are popular, but others — especially the mandate that virtually everyone have insurance coverage — are not.

Also, an estimated 26 million people will remain without health coverage once the law is fully implemented, including illegal immigrants, people who don't sign up and elect to face the fine instead, and those who can't afford it even with the subsidies.

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(Photo: JOSHUA ROBERTS /REUTERS/LANDOV)