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The conflict gave the Supreme Court a chance to try to spell out the federal government’s relationship with states and individuals more clearly than ever before, indicating where the federal government’s powers to tax, to regulate interstate commerce and to enact laws “necessary and proper” to exercise other powers began and ended.

The Roberts opinion strikes a new balance, putting limits on Congress‘ compulsory powers under the Commerce Clause but saying lawmakers retain wide authority under the taxing power.

Voters have been sharply polarized over the law since the beginning, and the court’s ruling is likely to dominate the presidential campaign as Mr. Obama tries to fend off Mr. Romney, who signed a pioneering health law as governor of Massachusetts.

Justice Ruth Bader Ginsburg’s concurring opinion cited that Massachusetts law as proving that an individual mandate would work in bringing down the number of uninsured people.