WASHINGTON — The US Supreme Court enshrined Barack Obama's legacy as a transformative president Thursday, but its fateful health care decision unleashed an explosive new row in an already volatile election.

A divided conservative majority court narrowly upheld the bulk of Obama's defining political act, a historic law that brought the world's richest nation closer than ever before to universal health coverage for all citizens.

"I know there will be a lot of discussion today about the politics of all this -- about who won and who lost," Obama said in a televised address.

"But that discussion completely misses the point."

But in Washington, being Washington, pundits spent the day assessing the impact of the ruling on Obama's November election clash with Republican Mitt Romney.

On the face of it, the ruling was a huge vindication for a president who expended his political capital to drive the reform through Congress even though critics said he should have focused solely on the economic meltdown.

Successive Democratic presidents had failed to pass health reform, which amounted to the most sweeping social reform legislation since the 1960s.

But the ruling also offered new ammunition for conservatives fiercely opposed to what they dub "Obamacare" and will ensure the unpopular law remains a key election issue, which may trouble the president.

"The guy who was most threatened today was Obama. There was almost no place for him to hide had he lost this -- the seminal act of his presidency... was going to go under," Stephen Ryan, former general counsel of the Senate Committee on Governmental Affairs, told AFP.

"It is as big an act as killing Osama bin Laden in terms of giving his presidency a temporary boost."

Republicans would have argued had the act been struck down that Obama spent a year-and-a-half fighting for a reform that Americans did not want, that detracted from reviving the economy and was not even constitutional.

Now, Obama can turn to Romney, who has vowed to repeal Obamacare, in a presidential debate, and say the act bears the approval of a conservative-majority Supreme Court.

"Politically, it is a clear win for President Obama, who can now campaign on the legitimacy of his signature issue come this fall," said Christopher Malone, a political science professor at Pace University in New York.

Senior Obama aides privately cautioned against the idea that Obama could expect a huge electoral boost at the polls from Thursday's ruling.

Costas Panagopoulos of Fordham University said the decision would "set the stage for the coming election, it gives the president some ammunition, and it gives his opponents some ammunition."

Though Chief Justice John Roberts infuriated fellow conservatives by siding with Obama he also offered them an opening, by arguing that the government could tax consumers who refuse to buy health insurance.

"What we now have is the biggest tax increase in the history of the world," said conservative talk show host Rush Limbaugh.

Romney tried to mine a similar seam, saying Obamacare, passed in 2011, was a symptom of Obama's wish for a "larger and larger government, more and more intrusive in your life," Romney said.

There were also signs that the Republican Party base, adamantly opposed to Obamacare, was becoming enflamed Thursday with the knowledge that the election is now the last chance to repeal health care reform.

The Romney campaign said it had received $2 million in donations within eight hours of the judgment being handed down.

Conservatives were in open revolt.

"President Obama believes he is above the law, entitled to abusing his power to get what he wants, and willing to violate the constitution and the oath he was sworn to uphold," said former Republican presidential candidate Rick Santorum.

"He has proven to be a very dangerous person to have this kind of power, and if he is not stopped this November, I am fearful that the make-up of this country as established by our founders will never be the same."

Obama appeared to recognize his continuing vulnerability on health care in his first reaction to the court's move.

"It should be pretty clear by now that I didn't do this because it was good politics. I did it because I believed it was good for the country," he said.

But the president also carefully listed the benefits he sees accruing from his reform effort.

"President Obama has one last chance to sell to the American people what the bill does, which he has never really successfully done before," said Evans.

In a knife-edge election, it may be that the furor of Thursday will be long forgotten when the election rolls around on November 6 and that the economy remains the over-riding issue.