WASHINGTON - The nine justices of the US Supreme Court are set on Tuesday to hear arguments on whether the Constitution provides same-sex couples the right to marry, taking up a contentious social issue in what promises to be the year's most anticipated ruling.
The decision, due by the end of June, will determine whether gay marriage will be legal nationwide. The arguments centre on gay marriage bans in Kentucky, Michigan, Ohio and Tennessee, four of the 13 states that currently prohibit it.
All eyes will be on conservative Justice Anthony Kennedy, who may cast the deciding fifth vote on a court closely divided on gay rights. The four liberal justices are expected to support same-sex marriage, and Kennedy has a history of backing gay rights. In decisions since 1996, Kennedy has broadened the court's view of equality for gays.
Kennedy authored a 5-4 decision in the most recent gay rights ruling, a 2013 rejection of a federal law defining marriage as between a man and woman for purposes of federal benefits.
Kennedy said the statute's only purpose was "to disparage and to injure those whom the state, by its marriage laws, sought to protect in personhood and dignity."
Public support for gay marriage has steadily grown in recent years and is particularly strong among younger Americans.
Before gay marriage became legal in the liberal northeastern state of Massachusetts in 2004, it was not permitted in any state. Now it is legal in 37 states and Washington, D.C.
The arguments, due to begin at 10 a.m. (1400 GMT), are divided into two parts.
The first, set for 90 minutes, is on whether the Constitution's guarantees of due process and equal protection under the law mean states must allow gay couples to marry.
The second, scheduled for an hour, concerns whether states must recognise same-sex marriages occurring out-of-state.
Gay rights activists call same-sex marriage a leading American civil rights issue of this era.
Opponents say same-sex marriage legality should be decided by individual states, not judges. Some opponents argue it is an affront to traditional marriage between a man and a woman and that the Bible condemns homosexuality.
President Barack Obama is the first sitting president to support gay marriage. His administration will argue on the side of same-sex marriage advocates.
The decision will affect not just the right of gay people to marry but also their right to be recognised as a spouse or parent on birth and death certificates and other legal documents.