The U.S. Supreme Court upheld the core of President Barack Obama's health-care overhaul, giving him an election-year triumph and preserving a law that would expand insurance to millions of people and transform an industry that makes up 18 percent of the nation's economy.

The justices, voting 5-4, said Congress has the power to make Americans carry insurance or pay a penalty. That requirement is at the center of the law, which also forces insurers to cover people with pre-existing medical conditions. The court modified the law's extension of the Medicaid program for the poor by saying the federal government can't threaten to withhold existing money from states that don't fully comply.

The ruling frames the health-care issue for this year's elections and is a victory of symbolism as well as substance for Obama. Chief Justice John Roberts, a Republican appointee, joined four Democratic-selected justices to give Obama a majority on a law that has divided the country along ideological and partisan lines throughout his presidency.