[Updated at 10:52 a.m. ET]
Anti-gay protesters have also gathered in support of keeping DOMA in place. While some say they are concerned that same-sex marriage threatens the traditional family institution, others offered more fiery rhetoric.
[Updated at 10:39 a.m. ET]
The weather has warmed up a bit in Washington, and so has the crowd. Same-sex marriage supporters who want to see an end to DOMA are rallying outside the Supreme Court.
[Updated at 10:14 a.m. ET]
The arguments should be under way by now, but the high court isn't expected to rule on DOMA until sometime this summer.
So let's talk about what's at stake here: money. If the Supreme Court overturns the federal law that defines marriage as solely between a man and a woman, some married same-sex couples will save $8,000 or more in income tax, according to a new study.
A same-sex couple with combined income of $100,000, in which one person earns $70,000 and the other makes $30,000, currently pays an extra $1,625 per year by filing separately rather than jointly, according to an analysis H&R Block conducted for CNNMoney. The calculations assume a standard deduction, no children and no tax credits.
The extra tax liability jumps to nearly $8,000 when one spouse earns all $100,000 and the other reports no income. In this case, couples filing jointly owe tax of $11,858, while a same-sex couple filing separately owes $19,585 - a 65% difference.
[Updated at 10 a.m. ET]
If things are rolling according to plan, court is in session now and the oral arguments on DOMA will start after a few housekeeping items.
[Updated at 9:41 a.m. ET]
As the justices prepare to hear a second day of oral arguments, the crowds have returned outside the steps of the U.S. Supreme Court. The sun is out, the temperatures are down - but the flags are up.
Some protesters had parked themselves, braving near freezing temperatures at dawn, to secure a good seat.
Edith Windsor, the 83-year-old plaintiff in the DOMA case, greeted protesters earlier in the morning on her way into the courthouse.
[Updated at 9:15 a.m. ET]
Less than an hour to go before oral arguments start, but same-sex marriage is gaining steam on social media. As of 9 a.m. #DOMA and #SCOTUS were among the top five U.S. topics trending on Twitter -- not bad for #humpday.
[Updated at 8:34 a.m. ET]
Today's drill is similar to yesterday's -- arguments to start a little after 10 a.m. ET, and no cameras. But lawyers arguing before the court will get one hour and 50 minutes. Hey, that's almost an hour more than what Proposition 8 got.
In addition to Senior Legal Analyst Jeffrey Toobin, CNN's Joe Johns and Bill Mears will be inside the courthouse to bring us the latest.
[Updated at 8:25 a.m. ET]
To understand the arguments around DOMA, get to know Edith Windsor -- the woman at the heart of the case. "Edie," as she's well known, spent 42 years of her life with Thea Clara Spyer.
But even after they married in 2007 in Toronto, four decades into their courtship, the two women were not "like most couples" in the eyes of the state of New York, where they lived, nor in the eyes of the U.S. government, which under the Defense of Marriage Act mandates that a spouse, as legally defined, must be a person of the opposite sex.
In 2009, a month after Spyer died, Windsor was slapped with a massive bill for inheritance taxes -- $363,053 more than was warranted, she later claimed in court -- because Spyer was, in legal terms, just a friend. Windsor would ultimately argue in court that her relationship with Spyer should not be considered any different than a heterosexual couple when it came to rights, taxes and other issues.
In October, Windsor, now 83, got an answer in the form of a ruling opinion from the 2nd U.S. Circuit Court of Appeals. That court found, in her favor, that the Defense of Marriage Act violates the Constitution's equal protection clause and thus she shouldn't have had to pay an inheritance tax after her partner's death. This follows a similar ruling, in May, from another federal appeals court in Boston.