The U.S. 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 U.S. law for making gay marriages 'unequal.' Scroll down for video . A moment in history: People line up outside the Supreme Court in Washington April 26 ahead of Tuesday's arguments focusing on gay marriage . 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 U.S. 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. 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 on Tuesday.
Top US court is slated to hear arguments in a gay marriage case Tuesday.
A majority vote could make gay marriage legal nationwide.
The court will publish a decision by June.