The law will bring near-universal coverage to a wealthy country in which tens of millions of people are uninsured. The plan's provisions will be phased in over four years, and it is expected to expand coverage to about 95 percent of eligible Americans, compared with 83 percent today.
"We have now just enshrined the core principle that everybody should have some basic security when it comes to their health," Obama said at a signing ceremony at the White House, where he was joined by House and Senate Democrats who backed the bill as well as ordinary Americans whose health care struggles have touched the president.
The plan will eventually extend coverage to 32 million uninsured Americans, reduce federal budget deficits and ban such insurance company practices as denying coverage to people with existing medical problems.
Republicans united in opposition pledged to repeal Obama's redesign of the health care system, which they criticized as a costly government takeover affecting one-sixth of the U.S. economy. The Republicans lack the votes needed to repeal the measure, but the minority party plans to use the issue to try to regain control of Congress in the November elections.
Shortly after Obama signed the bill, mostly Republican attorneys general from 13 states said they are suing the federal government to stop the health care overhaul, arguing that the provision that requires Americans to carry health insurance is unconstitutional. Experts say the effort will likely fail because the U.S. Constitution states that federal law supersedes state laws, but it will keep the issue alive until the November elections.
Democratic lawmakers say they have delivered on Obama's campaign pledge for change, revamping a system in which the spiraling costs have put health care and insurance out of the reach of many Americans.
Now the president must sell the law's merits to a wary American public.
The next act begins Thursday, when Obama visits Iowa City, Iowa, where he announced his health care plan as a presidential candidate in May 2007. There Obama plans to talk about how the new law will help lower health care costs for small businesses and families, selling the overhaul to Americans who are deeply divided over the plan.
The House passed the 10-year, $938 billion bill Sunday night after a rancorous debate. Not one Republican voted for the bill. Some Democrats also voted against it.
The measure represents the biggest expansion of the U.S. federal government's social safety net since the 1960s, when President Lyndon B. Johnson enacted the Medicare and Medicaid government-funded health care coverage programs for the elderly and poor.
A companion measure sought by House Democrats to make a series of changes to the main bill was approved 220-211. It goes to the Senate, where debate could begin as early as Tuesday. Majority Leader Harry Reid says he has the votes to pass it - though only under special budget rules requiring just 50 votes rather than the 60 usually needed to bypass the opposition's delaying tactics.
Republicans plan to offer scores of amendments to slow or change the companion measure, which Democrats hope to approve as written and send directly to Obama for his signature.
Even so, the health care debate will likely continue for months as both parties try to use it to motivate their backers to turn out in huge numbers in the November congressional elections.
Obama's yearlong campaign to overhaul health care seemed at a dead end in January, when a Republican won a special election to fill the late Edward Kennedy's Massachusetts Senate seat, and with it, enough votes to prevent the bill from coming to a final vote in the Senate. But the Democrats regrouped and soon came up with a rescue plan that required the House to approve the Senate-passed measure despite opposition to many of its provisions, then have both chambers pass a measure incorporating numerous changes.
Obama has pushed health care as his top priority since taking office in January 2009. Failure would have weakened him and endangered other issues on the president's ambitious domestic agenda, including immigration reform and climate change legislation.
So-called tea party activists, who believe that government spending and influence should be limited, are vowing to exact political revenge on those who passed it.
The government "has declared war on our way of life," activist Eric Odom from Nevada said Monday.
In Florida, about 85 tea party groups have about 100,000 members, according to Everett Wilkinson, a leader in the state's movement. A rally was planned in Boca Raton later Tuesday.
The movement's name is taken from the Boston Tea Party, a 1773 protest in which activists in the then-British colonies in America boarded ships and threw their cargo of English tea into Boston Harbor in a symbolic act of protest against taxes.