The US Supreme Court's arguments on Tuesday over same-sex marriage will cap more than two decades of litigation and a transformation in public attitudes.

Based on the court's actions during the past two years, a sense of inevitability is in the air: that a majority is on the verge of declaring gay marriage legal nationwide.

Justice Anthony Kennedy, the court's pivotal member on gay rights, has been marching in this direction with opinions dating to 1996. In his most recent gay rights decision for the court in 2013, rejecting a legal definition of marriage limited to a man and woman for purposes of federal benefits, Kennedy deplored that US law for making gay marriages "unequal."

That 5-4 decision did not address a constitutional right to same-sex marriage, but lower court judges interpreted the ruling as an endorsement of it and began invalidating state bans.

When states appealed rulings striking down their same-sex marriage prohibitions, the Supreme Court declined to intervene, most notably in October 2014 when it denied appeals in seven cases on a single day.

Instead, the nine justices are hearing in Tuesday's oral arguments an appeal of the sole decision from a regional US appeals court that went the opposite way. Last November, the Cincinnati-based U.S. Court of Appeals for the 6th Circuit upheld gay marriage bans in Ohio, Michigan, Kentucky and Tennessee.

With 37 of the 50 states now permitting gay marriage, many because of judicial orders, it seems unlikely the country's highest court would reverse course. Public opinion polls over the last decade have shown large increases in support for gay marriage. A ruling is due by the end of June.

KEY SWING VOTE

Yet some questions remain.

How much will Kennedy, a member of the court's five-man conservative bloc who often casts decisive votes in close cases, show his hand in the 2-1/2 hours of oral arguments? Will he reveal a clear view that the Constitution gives gay people a right to marry or will he voice concerns for state interests in controlling marriage laws?

An element of uncertainty hovers over Chief Justice John Roberts, who broke with the other court conservatives and cast the deciding vote upholding President Barack Obama's healthcare law in 2012. Roberts voted against gay rights in the 2013 ruling. But he separated himself from the most conservative dissenters and declined to declare outright that states may ban gay marriage.

He has demonstrated apprehension about the reputation of the court that, by virtue of his service as chief justice, informally bears his name. In his opinions, he has sometimes tried to lower tensions in controversial cases and reassure people that the court is aligning with precedent and public expectations.

The question is not only how Roberts might vote but what he might write.

In the 2013 ruling, he denounced the court majority's sentiment that federal lawmakers were deliberately harming gay people with the limited definition of marriage. "I would not tar the political branches with the brush of bigotry," he wrote.

For the other seven justices, expectations are clearer.

The four liberals, Ruth Bader Ginsburg, Stephen Breyer, Sonia Sotomayor and Elena Kagan, have signaled their opposition to state same-sex marriage bans. On the other side have been the three most conservative justices, Antonin Scalia, Clarence Thomas and Samuel Alito, asserting that nothing in the Constitution guarantees same-sex marriage.

Two legal questions are before the justices: whether the Constitution's guarantees of due process and equal protection cover a right to same-sex marriage; and, if they do not, whether states that ban same-sex marriages must recognize such unions performed in other states.

Gay couples and their families, about 30 adults and 20 children, have appealed the 6th Circuit's decision. The name petitioner is James Obergefell, who wanted his home state of Ohio, which prohibits gay marriage, to recognize his Maryland marriage to John Arthur as Arthur was dying from amyotrophic lateral sclerosis, also known as Lou Gehrig's disease.

Officials expect the courtroom to be packed to its 400-seat capacity. Lines for general spectator seats began forming around 6am on Friday, more than four days ahead of the scheduled oral arguments at 10am (1400 GMT) on Tuesday.