(Reuters) - President Barack Obama took an opening shot at conservative justices on the Supreme Court on Monday, warning that a rejection of his sweeping healthcare law would be an act of "judicial activism" that Republicans say they abhor.
Obama, a Democrat, had not commented publicly on the Supreme Court's deliberations since it heard arguments for and against the healthcare law last week.
Known as the "Affordable Care Act" or "Obamacare," the measure to expand health insurance for millions of Americans is considered Obama's signature domestic policy achievement.
A rejection by the court would be a big blow to Obama going into the November 6 presidential election.
Republican presidential candidates, who are vying to take on Obama in November elections, have promised to repeal the law if one of them wins the White House.
Obama's advisers say they have not prepared contingency plans if the measure fails. But the president -- who expressed confidence that the court would uphold the law -- made clear how he would address it on the campaign trail if the court strikes it down.
"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 at a news conference with the leaders of Canada and Mexico.
Conservative leaders say the law, which once fully implemented will require Americans to have health insurance or pay a penalty, was an overreach by Obama and the Congress that passed it.
The president sought to turn that argument around, calling a potential rejection by the court an overreach of its own.
"And I'd just remind conservative commentators that, for years, what we have heard is, the biggest problem on the bench was judicial activism, or a lack of judicial restraint, that an unelected group of people would somehow overturn a duly constituted and passed law," Obama said.
"Well, this is a good example, and I'm pretty confident that this court will recognize that and not take that step," he said.
The Supreme Court justices are expected to issue decisions in the dispute by late June, a time when the presidential campaign season is likely to be in full swing.
"It's not that common for presidents to get into direct verbal confrontations with the Supreme Court," said Georgetown University law professor Louis Michael Seidman. "But it's also not that common for the Supreme Court to threaten to override one of the president's central legislative accomplishments."
A spokeswoman for the court declined to comment on Obama's remarks.
A spokeswoman for Mitt Romney, the front-runner for the Republican presidential nomination, took issue with Obama's preemptive strike and his use of the word "unprecedented."
"What was ‘unprecedented' was the partisan process President Obama used to shove this unconstitutional bill through despite the overwhelming objections from Americans across the country," said Romney spokeswoman Andrea Saul.
"Even if the law is upheld, Governor Romney will begin the process of repealing it on Day One in office."
Romney shepherded healthcare reform through the state of Massachusetts when he was governor there. Democrats note that Romney's law was an inspiration for Obama's.
The president, who once taught constitutional law at the University of Chicago, said the "individual mandate" that requires most people to buy insurance was critical to the success of the healthcare overhaul.
The Supreme Court is looking at whether Congress exceeded its power to regulate commerce in U.S. states with that mandate.
"I think the justices should understand that in the absence of an individual mandate, you cannot have a mechanism to ensure that people with preexisting conditions can actually get health care," Obama said.
"So there's not only a economic element to this, and a legal element to this, but there's a human element to this. And I hope that's not forgotten in this political debate."
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.
Though Obama said he was confident the court would uphold the law, the centerpiece of his political legacy, he appeared to be previewing campaign trail arguments should the nine justices strike the legislation down.
In a highly combative salvo, Obama also staunchly defended the anchor of the law -- a requirement that all Americans buy health insurance -- as key to giving millions of people access to treatment for the first time.
"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.
Pointed comments from Supreme Court justices last week during three days of compelling hearings have convinced many commentators that the court, expected to rule in June, will declare the law, dubbed ObamaCare, unconstitutional.
Such a move would electrify the White House race, puncture Obama's claims to be a reformer in the grand political tradition, and throw the US health care industry into chaos.
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 in his first comments on last week's hearings.
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 health care case is the most closely watched Supreme Court deliberation since a divided bench handed the 2000 presidential election to George W. Bush over Al Gore, and could have far reaching political implications.
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.
Opponents of the health care law argue that the government has overreached its powers by requiring all Americans to purchase health insurance.
But supporters say that the government is within its rights to regulate the health industry as it has the power to oversee commerce across state borders.
Without the mandate, they say, the costs of insuring an extra 32 million Americans would be prohibitive to the private health insurance industry.
The Affordable Care Act is highly polarizing in US politics as the election approaches and Obama is yet to get a political dividend for the huge expenditure of political capital required to pass the legislation.