It's one that will transcend his presidency, change America's social safety net and shape how he is likely to be remembered.
Then there's the catch.
If Obama does not win a second term in November, he risks losing both the law and the core of his legacy. Republican Mitt Romney will try to gut the law and impose something else. All the rest of what Obama accomplishes will fall under the dimmer view of history assigned to one-term presidents.
Immediate attention isn't on the lasting consequences. Right now, the campaign retains its focus and remains a biting contest between two men with vastly different visions about how to fix the economy.
Obama's re-election message is not expected to differ because of the ruling. But his presidency has changed.
Where others failed, he succeeded, pushing through a plan to get basic health coverage to millions of uninsured people in the richest nation on earth.
"Obamacare," as critics derisively call it and supporters adoringly do, is his Medicare, his Social Security.
The high court ensured that the law would crown Obama's legacy. He did it with no Republican help in Congress, with half the country against him, with a Supreme Court led by a conservative chief justice who produced the surprising, deciding vote to rescue his law.
"If he wins re-election, he's got one huge marker down that he can build upon," said Bill Daley, Obama's former chief of staff.
Daley said if Obama achieves a long-term debt deal on immigration or brokers an energy plan as well, and avoids major mess-ups for eight years, "then that has the potential to be a very significant presidency."
Douglas Brinkley of Rice University, among the presidential historians who have met with Obama for dinners, said Obama wants to be remembered on the scale of Theodore Roosevelt, Franklin D. Roosevelt and Lyndon Johnson.
Now, Brinkley said, Obama is poised to go down as one of the leading progressive presidents for delivering on a health care promise that has eluded so many, for so long.
"It's a bit of confounding presidency," Brinkley said. "It always seems like Obama is about to flip off the rails. And then lo and behold, he's back on top again."
And if he loses to Romney? "It all changes," Brinkley said. "One-term presidents have a hard time building tremendous legacies."
The law is built around a mandate that people who can afford to buy health insurance must do so to help rein in the costs of coverage for everyone. Overall, the legislation is unpopular with the public, although individual parts of it are not.
Romney calls it an act of government intrusion and says he will ask Congress to repeal the law. Obama's triumph at the Supreme Court, therefore, seemed less about legislative permanency and more about electoral urgency.
Within hours, a fundraising appeal under Obama's name warned donors of Romney's undo-it plans and said, "We can't allow that to happen. We have to win this election."
Just don't expect Obama's fundamental pitch to voters to turn much at all, White House and campaign officials said.
To the degree that the health care law becomes a more prominent part of his campaign, it will still be in the framework that Obama has settled upon, his vision of economic revival and opportunity and fair taxation.
He wins by persuading people about the potential jobs ahead, not the health care fight behind him.
Campaign officials say they do see some fresh opportunity to change perceptions about the law and draw contrasts with Romney. They will do that by focusing on popular elements that could disappear, such as the promise of insurance for people with medical conditions.
In his majority opinion, Roberts made clear he was not offering a policy endorsement of what Obama and his Democratic allies had done.
"Those decisions are entrusted to our nation's elected leaders, who can be thrown out of office if the people disagree with them," the chief justice said. "It is not our job to protect the people from the consequences of their political choices."
Obama is that guy trying not to get thrown out of office. He had an eye on the legacy of the law within hours of knowing it was saved.
"I'm as confident as ever that when we look back five years from now, or 10 years from now, or 20 years from now, we'll be better off, because we had the courage to pass this law and keep moving forward," he said.
Implied in there was his message to voters to keep moving forward with him.
"The court victory is a huge step towards making affordable health care a reality to everyone in the country," said Robert Gibbs, a senior advisor to Obama's campaign. "We have another big hurdle left. It's called the election."