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President Obama, employing his strongest language to date on the Supreme Court review of the federal health care overhaul, cautioned the court Monday against overturning the law -- while repeatedly saying he's "confident" it will be upheld.

The president spoke at length about the case at a joint press conference with the leaders of Mexico and Canada. The president, adopting what he described as the language of conservatives who fret about judicial activism, questioned how an "unelected group of people" could overturn a law approved by Congress.

"I'm 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 Supreme Court spent three days hearing arguments last week in four separate challenges to the health care law, which stands as the president's signature domestic policy accomplishment. A central challenge was over the individual mandate -- the requirement that Americans buy health insurance. Critics say the mandate is unconstitutional, and that the federal government cannot force people into the insurance marketplace.

Obama on Monday said that without such a mandate, the law would not have a mechanism to ensure those with preexisting conditions get health care.

"I'm confident that this will be upheld because it should be upheld," Obama said, describing the law as "constitutional."

Republican lawmakers slammed the president for his Supreme Court comments. Sen. Orrin Hatch, R-Utah, accused the president of misrepresenting the implications of a ruling against the law.

"It must be nice living in a fantasy world where every law you like is constitutional and every Supreme Court decision you don't is 'activist,'" he said in a statement. "Many of us have been arguing for nearly three years that the federal government does not have the power to dictate individuals' purchasing decisions. After a national debate on the subject, more than two-thirds of Americans agree that the Obamacare insurance mandate is unconstitutional."

The president spoke following meetings with Mexican President Felipe Calderon and Canadian Prime Minister Stephen Harper. Until the heath care case remarks, the press conference was focused mostly on economic issues, as well as the war on drugs.

The leaders vowed a new effort to boost North American trade and cut needless regulation that stifles it. "Our three nations are going to sit down together, go through the books and simplify and eliminate more regulations that will make our joint economies stronger," Obama said.

Obama noted trade among the three neighbors now tops $1 trillion a year, and he wants to see that number rise.

But notable by its absence from the post-summit news conference in the Rose Garden was the controversial Keystone XL oil pipeline from Canada's oil sands in Alberta to the U.S. Gulf of Mexico. Obama shelved the plan pending further review -- and has endured ferocious GOP attacks ever since, with Republicans calling the move a blow to job creation and U.S. energy needs. He maintains GOP leaders in Congress forced his hand by insisting on a decision before an acceptable pipeline route was found.

Harper has voiced disappointment with Obama's decision. He also visited China in February to explore alternatives. Canada has the world's third-largest oil reserves -- more than 170 billion barrels -- after Saudi Arabia and Venezuela, and daily production of 1.5 million barrels from the oil sands is expected to rise to 3.7 million by 2025.

The Associated Press contributed to this report.