Conservative U.S. Supreme Court justices on Tuesday signaled skepticism over the legality of President Joe Biden’s plan to cancel $430 billion in student debt for about 40 million borrowers, with the fate of his policy that fulfilled a campaign promise hanging in the balance. The nine justices heard arguments in appeals by Biden’s administration of two lower court rulings blocking the policy that he unveiled last August in legal challenges by six conservative-leaning states and two individual student loan borrowers opposed to the plan’s eligibility requirements.