President Barack Obama took an opening shot at conservative justices on the U.S. 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 down the law.

"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 formal opinions on the subject 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," saidGeorgetown University law professor Louis Michael Seidman. "But it's also not that common for the Supreme Courtto threaten to override one of the president's central legislative accomplishments."

A spokeswoman for the court declined to comment on Obama's remarks.