President Obama issued a rare, direct challenge Monday to the Supreme Court to uphold his historic health care overhaul, weighing in with a vigorous political appeal for judicial restraint. He warned that overturning the law would hurt millions of Americans and amount to overreach by the “unelected’’ court. Obama predicted that a majority of justices would uphold the law when the ruling is announced in June. But the president, himself a former law professor, seemed intent on swaying uncertain views in the meantime, both in the court of public opinion and in the minds of the justices, about not overstepping the high court’s bounds.