WASHINGTON — US President Barack Obama on Monday challenged the "unelected" Supreme Court not to take the "extraordinary" and "unprecedented" step of overturning his landmark health reform law.

In a highly combative salvo on a case which could have a critical impact on his reelection chances, Obama warned that health care for millions of people was at stake, even as nine Supreme Court justices deliberate the arguments.

"Ultimately, I am confident that the Supreme Court will not take what would be an unprecedented, extraordinary step of overturning a law that was passed by a strong majority of a democratically elected Congress," Obama said.

The US Supreme Court held compelling legal arguments on the health reform law, the centerpiece of Obama's political legacy, last week, amid a flurry of commentary predicting the law will ruled unconstitutional.

Obama noted that for years, conservatives had been arguing that the "unelected" Supreme Court should not adopt an activist approach by making rather than interpreting law, and held up the health legislation as an example.

"I am pretty confident that this court will recognize that and not take that step," Obama said during a press conference in the White House Rose Garden with the leaders of Canada and Mexico.

Following pointed questions by Supreme Court justices to the government's lawyer at hearings on the health care case last week, many commentators are predicting a conservative majority on the court will strike the law down.

Obama's comments will be seen as a warning shot to the court, one of the three branches of the US government, and could draw complaints from critics that he is trying to influence the deliberations.

The president also vigorously defended the law's individual mandate clause by which the government requires all Americans to buy health insurance coverage to expand coverage and in return for requiring insurers to cover all patients.

"I think the justices should understand, in the absence of an individual mandate, you cannot have a mechanism to ensure that people with pre-existing conditions can actually have health care."

Obama also argued there was a "human element" to the health care battle, as well as legal and political dimensions.

He said that without the law, passed after a fierce battle with Republicans in 2010, several million children would not have health care, and millions more adults with pre-existing conditions would also be deprived of treatment.