This was all set in motion in the heat of the president's re-election campaign in what government sources described as a quickly arranged political strategy.
"At a certain point, I've just concluded that for me personally it is important for me to go ahead and affirm that I think same-sex couples should be able to get married," Obama told ABC News.
Perhaps bowing to political caution in an election year, Obama explained he "hesitated on gay marriage" partly because he thought civil unions "would be sufficient."
"I was sensitive to the fact that for a lot of people, the word marriage was something that invokes very powerful traditions and religious beliefs," he said.
A new national survey indicates a small majority of Americans backs such marriages but there are generational and partisan divides as well as a gender gap.
According to a CNN/ORC International poll, 53 percent of the public think marriages between gay or lesbian couples should be legally recognized as valid, with 44 percent opposed.
"There are big differences among younger and older Americans, with the youngest age group twice as likely than senior citizens to support same-sex marriage," says CNN Polling Director Keating Holland. "Women are also more likely to call for legal recognition of gay marriage than men. And only three in ten Americans who attend religious services every week support same-sex marriage while six in ten Americans who don't attend church weekly feel that way."
Former President Bill Clinton, who signed the marriage law into effect 17 years ago, said this month he now backs the right of homosexuals to marry. So, too, does his wife, former Secretary of State Hillary Clinton, a possible Democratic presidential candidate in 2016.
But others on the right say society benefits from preserving a view of marriage that has been in place for centuries.
The authors of a new book, "What is Marriage? Man and Woman: A Defense," say, "Redefining marriage would, by further eroding its central norms, weaken an institution that has already been battered by widespread divorce, out-of-wedlock child bearing and the like."
Academics Robert George, Sherif Girgis, and Ryan Anderson say marriage should be more than "commitment based on emotional companionship," and has been practiced for a specific reason throughout this country's history.
"All human beings are equal in dignity and should be equal before the law. But equality only forbids arbitrary distinctions," they argue. "And there is nothing arbitrary about maximizing the chances that children will know the love of their biological parents in a committed and exclusive bond. A strong marriage culture serves children, families and society by encouraging the ideal of giving kids both a mom and a dad."
The DOMA and Prop 8 cases will test the delicate line over constitutional limits from both congressional, presidential, and judicial power.
All nine justices will have an equal voice in both oral argument and an eventual ruling, but it may be one member of the court whose views may count most in the high-stakes quest for five votes.
Justice Anthony Kennedy authored the landmark 1996 Romer v. Evans ruling, striking down a Colorado constitutional amendment that forbid local communities from passing laws banning discrimination against gays.
The moderate-conservative wrote for the 6-3 majority, rejecting the state's argument the law only blocked gay people from receiving preferential treatment or "special rights."
In dissent Justice Antonin Scalia criticized the court for placing what he said was "the prestige of this institution behind the proposition that opposition to homosexuality is as reprehensible as racial or religious bias."
In 2003, Kennedy authored the court's decision overturning state laws criminalizing homosexual sodomy.
At the time though, he cautioned the court was not endorsing the idea of same-sex marriage, saying the private conduct case at hand "does not involve whether the government must give formal recognition to any relationship that homosexual persons seek to enter."
That caution was further articulated this month by Kennedy, when he said some issues are best left to the other branches.
"I think it's a serious problem. A democracy should not be dependent for its major decisions on what nine unelected people from a narrow legal background have to say," Kennedy said.
By patiently letting legislatures and the voters decide the social and practical implications of same-sex marriage over the past decade, the high court is now poised to perhaps offer the final word on tricky constitutional questions.
The split 5-4 conservative-liberal bench has the option of ruling broadly or narrowly-- perhaps taking a series of incremental cases over a period years, building political momentum and public confidence in the process.
The Prop 8 case is Hollingsworth v. Perry (12-144). The DOMA case is U.S. v. Windsor (12-307).