The Supreme Court heard opening arguments today from opponents and supporters of same-sex marriage in what will be their most decisive ruling on gay marriage since striking down the Defense of Marriage Act in 2013.

The case being argued before the court is whether individual states have the power to ban same sex marriage. The debate focuses on the states’ right to define marriage as being between one man and one woman, and whether marriage is a constitutional right.

If the Supreme Court finds that states do not have the right to define marriage, the 13 states currently enforcing gay marriage bans will be forced to reverse course and recognize same-sex unions.

The ruling is expected to be a close decision. The four conservative judges on the bench have made it clear early on that they will uphold the states’ right to define marriage. The courts liberal members, meanwhile, have reaffirmed their position that any ban on gay marriage is unconstitutional.

Justice Anthony M. Kennedy is expected to cast the deciding vote, as he did in the reversal of DOMA. He has not explicitly stated whether he is prepared to defend marriage as a constitutional right as fervently as he did in 2013.

John J. Bursch, an attorney arguing for the state of Michigan, framed the issue as one that should be decided by the democratic process, not a Supreme Court decision.

“This case is not about the best marriage definition,” Bursch said. “It is about the fundamental question regarding how our democracy resolves such debates about social policy.”

The Supreme court is expected to vote on the issue in June.