It's on.

The debate over same-sex marriage moves to the Supreme Court on Tuesday, where justices will hear arguments about whether there's a constitutional right to such unions nationwide.

The stakes are enormous. Thousands of couples across the country have married this year after a string of lower court rulings tossed out same-sex marriage bans in several states. A loss at the Supreme Court would be a serious blow to the gay rights movement, which has enjoyed a stunning set of victories in recent years.

Meanwhile, the case is becoming a major issue in the 2016 presidential contest, with Republicans balancing the rapidly changing politics surrounding same-sex marriage against the party's conservative base.

A decision isn't expected until June, but Tuesday's arguments could provide important insight into how the justices may vote. Here are five things to watch:

1. The arguments: Democratic process vs. the courts

A key part of the case is whether the decision should be left to the courts or to the citizens of each state. Court observers will be looking for clues about how the justices will come down on this issue.

Fourteen couples and two widowers are challenging state bans on same-sex marriage. They're represented by Mary L. Bonauto and Doug Hallward-Driemeier, who will argue that the freedom to marry is fundamental for all people and it's not an issue that should be up for popular vote.

They will stress that a ban \"stigmatizes and humiliates adults\" and \"deprives children of the protections of having two married parents.\"

The Obama administration supports the same-sex couples. Solicitor General Donald B. Verrilli will emphasize that the bans violate constitutional provisions that guarantee citizens equal protection under the law. In court briefs, he told the justices the bans send the \"inescapable message that same-sex couples and their children are second-class families, unworthy of the recognition and benefits that opposite-sex couples take for granted.\"

Two lawyers, John J. Bursch and Joseph Whalen, will represent the four states arguing to uphold their bans on same-sex marriage: Ohio, Kentucky, Michigan and Tennessee.

They will tell the justices the case is not about the definition of marriage. Instead, they will argue the case is about who gets to decide changes to marriage, the states or federal courts. They argue that because the Constitution is silent on the issue, it should be decided by the people in the states.

2. Justice Kennedy -- and maybe Chief Justice Roberts

It's dangerous to put too much weight on any particular exchange between lawyers and justices during Supreme Court oral arguments.

But once again, all eyes will be on Justice Anthony Kennedy, who has written a series of major gay rights opinions and could again provide a crucial vote.

That was the case in 2013 when he was pivotal in striking down a key provision of the Defense of Marriage Act, a law that denied federal benefits to lawfully married same-sex couples. In the 5-4 opinion, Kennedy said states have a historic authority to define marriage, language that lawyers defending state bans will likely cite to back up their position.

But Kennedy also relied upon arguments concerning equal protection in the DOMA case, something that challengers hope will be his focus again on Tuesday.

Court watchers will also pay close attention to Chief Justice John Roberts. He dissented in the DOMA case and is considered a likely vote to uphold the state bans. But he has never directly opined on the issue and gay marriage advocates hope he might consider a vote overturning the bans as part of his legacy as the leader of the Roberts Court.

3. The handling of the second question

Most of the attention has gone to the heart of the case: whether states can ban same-sex marriage.

But there is a second question the justices will consider that looks at whether states must recognize a lawful same-sex marriage that was performed in another state.

That's the question that relates to Jim Obergefell, the lead plaintiff in the case. In 2013, he traveled from Ohio to Maryland to marry John Arthur because of Ohio's ban on same-sex marriages. After Arthur died, Obergefell sued so the death certificate could be amended to reflect that he was married at the time of his death.

Some supporters of gay rights fear that the second question might provide an off ramp to justices who may not be ready to make a nationwide ruling.

4. Explaining the purpose of same-sex marriage bans

Watch carefully to see how both sides describe the purpose behind same-sex marriage bans.

This has been an important issue in the past, especially for Kennedy. In the DOMA case, for example, Kennedy objected to the law in part because he found its \"avowed purpose\" was to \"impose a disadvantage\" and \"stigma\" upon lawfully married same-sex couples.

Lawyers arguing on behalf of state bans will argue those laws weren't passed with any \"animus\" toward same-sex families. They will argue that marriage developed as an institution to \"encourage individuals with the inherent capacity to bear children to enter a union that supports raising children,\" and not with the intent to discriminate.

But Roberta A. Kaplan, who argued and won the DOMA case, disagrees. She has filed a friend-of-the-court brief supporting the Obergefell challengers, arguing the state's justifications are \"nothing more than pretexts for discrimination rooted in stereotypical thinking about a disfavored group.\"

5. The outside views that the justices might cite

More than 100 friend-of-the-court briefs have been filed on both sides of the case.

These briefs are meant to call the court's attention to a particular issue. Sometimes a justice seizes on an amicus brief at oral argument and catches a lawyer off guard. It's doubtful the justices have read all the briefs, but their clerks have certainly highlighted important issues.

The briefs range from those filed by legal scholars and law professors on a certain area of the law to others filed on behalf of state legislators. Kenneth B. Mehlman, the former chairman of the Republican National Committee, filed a brief on behalf of political moderates and conservatives including Mary Cheney, Rudolph W. Giuliani and John C. Danforth urging the justices to overturn same-sex marriage bans. Other briefs come from a wide variety of groups, including the Cleveland Choral Arts Association, the Survivors of Sexual Orientation Change Therapies, and Concerned Women for America.

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