US President Barack Obama has warned that a rejection of his healthcare reform by the US Supreme Court would amount to "judicial activism".
In a rare challenge to the US high court, Mr Obama said overturning the law would hurt millions of Americans.
The nine Supreme Court justices are currently considering whether parts of the controversial legislation violate the American constitution.
They are expected to announce their ruling in June.
Mr Obama said in a White House press conference on Monday that he was confident they "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".
He added: "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.
"Well, this is a good example. And I'm pretty confident that this court will recognise that and not take that step."
He also urged critics of the reforms not to forget the "human element" of the debate.
The BBC's Steve Kingstone in Washington says it is rare for a president to speak so plainly about a pending decision, and that his doing so is a sign of just how high the stakes are.
The Supreme Court spent three days last week considering a Republican-led legal challenge by 26 states to the law, which was approved by Congress two years ago.
The bill, which aims to extend health insurance to more than 30 million Americans, is the biggest overhaul of US healthcare in almost half a century.
The act's requirement that all those eligible should have medical cover has been condemned as an assault on civil liberties by conservatives.