Hearing arguments in a case that could destroy the framework of the Affordable Care Act, Supreme Court justices seemed torn Wednesday about whether the law provides tax credits to people in all states buying health insurance on the individual market.

The court spent nearly 90 minutes engaging attorneys about a single phrase in the 2010 law — a phrase that is backed up with so little legislative history it puts justices essentially in the position of guessing what Congress meant.

The justices’ reading of that phrase — “an Exchange established by the State” — will determine whether low- and moderate-income people in Oklahoma and 36 other states get federal help to buy insurance they might not otherwise be able to afford.

This is the third major Supreme Court case involving the Affordable Care Act — the second case on which its survival may depend. In 2012, the court ruled that the individual mandate was constitutional. But justices struck down the requirement that states expand Medicaid. Last year, the court ruled that closely held corporations don’t have to provide coverage for contraceptives.