The lengthy fight to allow gay marriage across America may soon be at an end with the Supreme Court set Tuesday to consider whether same-sex couples have a constitutional right to wed.
Already legal in 37 of the country's 50 states and in the capital Washington, experts say it seems inevitable that the nation's top court will recognize gay marriage.
"Marriage equality has advanced with remarkable speed through the states," said constitutional law professor Steven Schwinn of the John Marshall Law School in Chicago.
"In effect, the Supreme Court allowed same-sex marriages to move forward in those jurisdictions, making it all the more difficult to roll back marriage equality now."
As early as Friday afternoon, a line was forming outside the courthouse in Washington, with people hoping to secure a spot in the public gallery for the historic court session.
Schwinn said the court would likely vote in favor of gay marriage and the case "is almost sure to be a capstone in establishing federal civil rights for gays and lesbians."
The nine justices will hear from plaintiffs from four states -- Ohio, Michigan, Tennessee and Kentucky -- where gay marriage is still banned.
Supported by the Obama administration, the 16 plaintiffs want to legally marry but their home states define marriage as being between a man and a woman and do not recognize gay marriages carried out elsewhere in the country.
If the Supreme Court rules on these four states, it will be making a de facto decision on all the 13 states banning gay marriage.
At issue is the Supreme Court's interpretation of the 14th Amendment of the US Constitution, which provides equal protection under the law.
Judges must decide if this amendment means states must allow gay marriage, and whether states are required to recognize same-sex marriages that were conducted in other states.
"It's more likely that the court says yes to both questions or no to both questions, between those two choices it's certainly more likely they would say yes to both," said David Cruz, a professor from the University of Southern California Law School.
Maureen Holland, the lawyer for a plaintiff couple from Tennessee, said: "Same-sex marriage is... marriage, and should be respected by unanimous states."
She said "all indicators" suggest the court will rule in favor of gay marriage.
In a landmark decision in June 2013, the court struck down a law denying federal benefits to homosexual couples. But it stopped short of legalizing same-sex marriage nationwide, leaving that question to the states -- even though the court traditionally protects federalist principles.
The four states at the center of this case are being backed by several religious and conservative groups, which argue they have a right to protect a traditional definition of marriage.
More than 130 arguments have been filed in the case, the majority of which are in support of the plaintiffs.
Following on from previous positions in favor of gay rights, conservative justice Anthony Kennedy is expected to join the four liberal judges in approving gay marriage at the national level.
A ruling is expected at the end of June.