WASHINGTON - Claiming a historic triumph that could define his presidency, a jubilant Barack Obama signed a massive, nearly $1 trillion health care overhaul on Tuesday that will for the first time cement insurance coverage as the right of every U.S. citizen and begin to reshape the way virtually all Americans receive and pay for treatment.

After more than a year of hyperpartisan struggle - and numerous near-death moments for the measure - Obama declared "a new season in America" as he sealed a victory denied to a line of presidents stretching back more than half a century. Democratic lawmakers cheered him on, giving the White House signing ceremony a rally-like atmosphere as they shouted and snapped photos with pocket cameras or cell phones.

Not everyone was cheering. The Democrats pushed the bill through Congress without GOP support, and the Republicans said Tuesday that those Democratic lawmakers would pay dearly in this November's elections. Opinion polls show the public remains skeptical, too, and Obama will fly to Iowa on Thursday for the first of a number of appearances that will be more like a continuing sales job than a victory lap.

Aside from the huge, real-life changes in store for many Americans, the White House hopes the victory - even as a companion Senate "fix-it" bill moves through the Senate - will revitalize an Obama presidency that has been all but preoccupied with health care for his first year and two months in office. Vice President Joe Biden was caught whispering a profanity as he exclaimed to the president what a big deal it was.

Indeed, the reshaping of one-sixth of the U.S. economy, to be phased in over several years, ranks among the biggest changes ever devised by Washington. That was a main complaint from Republicans who characterize the measure as a costly, wrongheaded government power grab. Obama and the Democrats portray it as literally a lifesaver for countless Americans.

The core of the massive law is the extension of health care coverage to 32 million who now lack it, a goal to be achieved through a complex cocktail of new mandates for individuals and employers, subsidies for people who can't afford to buy coverage on their own, consumer-friendly rules clamped on insurers, tax breaks, and marketplaces to shop for health plans.

The law's most far-reaching changes don't kick in until 2014, including a requirement that most Americans carry health insurance - whether through an employer, a government program or their own purchase - or pay a fine. To make that a reality, tax credits to help cover the cost of premiums will start flowing to middle-class families and Medicaid will be expanded to cover more low-income people. Insurers would no longer be allowed to deny coverage to people with health problems.

Among the new rules on insurance companies that take effect this year are banning lifetime dollar limits on policies and policy cancellations when someone gets sick. Insurers also will have to allow parents to keep children on their plans up to age 26. But a much-touted provision to protect children in poor health has a gap. Insurers would still be able to deny new coverage to kids with health problems until 2014, although they could no longer refuse to pay for a particular condition once the child is covered.

The changes are to be paid for with cuts in projected government payment increases to hospitals, insurance companies and others under Medicare and other health programs, an increase in the Medicare payroll tax for some, fees on insurance companies, drug makers and medical device manufacturers, a new excise tax on high-value insurance plans and a tax on indoor tanning services.

For seniors, the plan will gradually close the "doughnut hole" prescription coverage gap and improve preventive care. But it also will cut funding for popular private insurance plans offered through Medicare Advantage. About one-quarter of seniors have signed up for such plans, which generally offer lower out-of-pocket costs.

Democrats, led by Obama, celebrated a "new wind at our backs" from an achievement accomplished after more than a year of high tension and deep division - stretching back to shouted protests that interrupted lawmakers' town hall meetings on the subject last summer. Obama signed the measure less than two days after the cliffhanger final House vote in a rare Sunday night session.

"Our presence here today is remarkable and improbable," Obama said, his grin wider than any in recent memory. "With all the punditry, all of the lobbying, all the game-playing that passes for governing in Washington, it's been easy at times to doubt our ability to do such a big thing, such a complicated thing."

At a second celebration later, he said, "After a century of striving, after a year of debate, after a historic vote, health care reform is no longer an unmet promise. It is the law of the land."

The president now faces the task of selling to the public a bill that satisfies neither side of the political spectrum.

Liberals bemoan that a government-run plan to compete with private ones was shed from the legislation during bitter negotiations. Conservatives fear an expansion of government and costs they say will bankrupt the country, despite an estimate from the nonpartisan Congressional Budget Office that the law will cut federal budget deficits by an estimated $143 billion over a decade.