A beaming President Barack Obama on Tuesday signed a historic $938 billion health care overhaul that guarantees coverage for 32 million uninsured Americans and will touch nearly every citizen's life, presiding over the biggest shift in U.S. domestic policy since the 1960s and capping a divisive, yearlong debate that could define the November elections.

Celebrating "a new season in America" — the signature accomplishment of his White House so far and one denied to a line of presidents before him — Obama made the massive bill law with an East Room signing ceremony. He was joined by jubilant House and Senate Democrats as well as lesser-known people whose health care struggles have touched the president. Obama scheduled back-to-back events to mark the moment, with much of his White House audience, as well as hundreds of others, gathering at the Interior Department for Act II immediately after the signing.

"With all the punditry, all the lobbying, all the game-playing that passes for governing here in Washington, it's been easy at times to doubt our ability to do such a big thing, such a complicated thing, to wonder if there are limits to what we as a people can still achieve," Obama said, his remarks interrupted by applause after nearly every sentence. "We are not a nation that scales back its aspirations. We are not a nation that falls prey to doubt or mistrust. We don't fall prey to fear. We are not a nation that does what's easy. That's not who we are. That's not how we got here."

The president's victory lap proceeded even as Congress labored to complete the overhaul with a companion measure making changes to the main bill that were a condition of House Democrats' approval. Debate on that bill, also passed Sunday by the House, could begin Tuesday in the Senate.

Not everyone was cheering the new law.

Attorneys general from 13 states filed suit to stop the overhaul just minutes after the bill signing, contending the law is unconstitutional. Florida Attorney General Bill McCollum took the lead in the lawsuit, joined by colleagues from South Carolina, Nebraska, Texas, Michigan, Utah, Pennsylvania, Alabama, South Dakota, Louisiana, Idaho, Washington and Colorado. Other GOP attorneys general may join the lawsuit later or sue separately.

In Washington, Republicans remained firm in their opposition to the giant remake of the nation's health system, declaring it much too costly and unlikely to produce the results that Obama claims. The Republicans pledged to see Democrats punished in this fall's elections for approving the legislation over deep public skepticism.

"By signing this bill, President Obama is abandoning our founding principle that government governs best when it governs closest to the people," said House Minority Leader John Boehner, R-Ohio. "Never before has such a monumental change to our government been carried out without the support of both parties. This debate has fostered unprecedented division at a time when this nation needs to come together and address the serious challenges we face."

With that in mind, and with many of the law's most sweeping changes not to take effect for years, Obama emphasized the overhaul's most immediate impacts, including the ability of young adults to remain on their parents' health plans and a ban on insurers denying coverage to sick children.

"We have now just enshrined the core principle that everybody should have some basic security when it comes to their health," the president said.

The second, much larger event had an even more combative, campaign-like feel. Obama thanked the players from labor unions to grass-roots supporters who helped push the bill forward, and openly criticized Republicans for "still making a lot of noise about what this reform means."

"Look it up for yourself," he urged the public. "You don't have to take my word for it, you'll see it in your own lives."

The White House did everything possible to make sure Obama's appearances carried the day without competition. A planned announcement of the administration's new drug control policy by Vice President Joe Biden was called off, White House press secretary Robert Gibbs declined to hold his regular daily briefing for reporters, and all Obama's meetings were closed to coverage, including one with Israeli Prime Minister Benjamin Netanyahu.

The day was about more than celebration. It marked the launch of an aggressive sales job Obama will undertake to turn around public opinion on the legislation and help save Democrats — particularly those from conservative-leaning districts — who stand to suffer most in the fall elections from casting votes for the bill.

That effort continues Thursday when Obama visits Iowa City, Iowa, where as a presidential candidate he announced his health care plan in May 2007.

Obama's historic achievement was sealed late Sunday, when the House voted 219-212 — without a single Republican in favor — to send the 10-year bill to Obama. Passed by the Senate in December, the bill eventually will extend coverage to 32 million uninsured Americans and ban such insurance company practices as denying coverage to people with medical problems.

The House also passed the companion measure on Sunday, by a 220-211. Majority Leader Harry Reid, D-Nev., says he has the votes to pass it in his chamber — though only under special budget rules requiring just a simple majority vote. Republicans plan to offer scores of amendments to slow or change that bill and stymie Democratic hopes to see it approved as written and sent directly to Obama for his signature.

The first changes under the overhaul take effect by the end of September. Other changes would not kick in until 2014.

By then, most Americans will for the first time be required to carry health insurance — through an employer, through a government program or by buying it for themselves. Those who refuse will face penalties from the IRS.

Tax credits to help pay for premiums also will start flowing to middle-class working families with incomes up to $88,000 a year, and Medicaid will be expanded to cover more low-income people.

The nonpartisan Congressional Budget Office said the bill awaiting Obama's signature would cut federal budget deficits by an estimated $143 billion over a decade.

The second measure, which House Democrats demanded before agreeing to the first one, includes money to close a gap in Medicare prescription drug coverage over the next decade.