By MICHAEL FALCONE ( @michaelpfalcone )

NOTABLES

• THE CASE: The nation's highest court today will hear long-awaited arguments in the case of California's Proposition 8, the nation's best-known and most hotly contested ban on gay marriage. As ABC's Chris Good and Ariane De Vogue note, along with another case scheduled for Wednesday, in which the court will consider a challenge to the federal Defense of Marriage Act (DOMA) ban on gay marriage, the Prop 8 case likely will lead to at least one landmark decision to be cited by advocates and opponents of same-sex marriage for decades to come. The Supreme Court is slated to issue rulings by the end of June. The arguments on Prop. 8 is scheduled to begin at 10 a.m. ET. http://abcn.ws/13sJCwt

• THE PLAYERS: The court will hear arguments from the bipartisan duo of Ted Olson and David Boies, who argued before the court on opposite sides of the 2000 case over Florida's vote recount in the presidential election between George W. Bush and Al Gore. (Boies served as the top lawyer for Gore's campaign; Olson, for Bush's). The unlikely pair was brought together by gay-rights advocates seeking to overturn California's ban, which took the gay activist community by surprise when it passed in 2008. Californians voted 52 percent to 48 percent in favor of banning gay marriage under their state constitution in 2008 on the same day they voted Barack Obama into the White House by a margin of 61 percent to 37 percent for Sen. John McCain, R-Ariz. Charles J. Cooper, representing a group called Protectmarriage.com, will argue in favor of the ban. http://abcn.ws/13sJCwt

• THE SCENE: This morning activists waving American flags and blue banners bearing the Human Rights Campaign symbol lined the sidewalk in front of the Supreme Court, reports ABC's Sarah Parnass. Sporting gloves and puffy jackets, they displayed signs saying, "Marriage is a human right not a heterosexual privilege" and "the nation is ready for marriage equality." Joey Williamson has been out here for 49 hours and he has at least another 24 to go. He is here from New York City waiting to hear the arguments against the Defense of Marriage Act tomorrow. "Obviously these are hugely historic cases," Williamson said. He saw the state Supreme Court case back home in New York so tomorrow will wrap off a long saga for him. "We thought if we could see this case all the way to the end it would be super interesting." Williamson also has a personal stake in the case. He hopes that when he one day gets married in New York, the federal government will recognize it. WATCH ABC'S Terry Moran's "Good Morning America" report setting the table for today's oral arguments: http://abcn.ws/XDERIs

THE ROUNDTABLE

ABC's RICK KLEIN: In 2003, when the first state high court acted to legalize gay marriage, the judiciary was clearly moving faster than the public. But inside a decade, as the Supreme Court hears the first of two cases today on the subject, the nation is more than ready for its top judges to weigh in. As polls have shown, and as politicians have realized (is it even news anymore when a Democrat reverses his or her longstanding opposition to gay marriage?), the country is moving with astounding speed on the issue. That's why, whatever the outcome of rulings on Prop 8 and DOMA, it's rulings on Prop 8 and DOMA - not a muddle based on standing - that all sides need from the high court. What's wrong with some legal clarity, in a debate that's moved this far, this fast?

ABC's GARY LANGER: The ABC News-Washington Post released last week tracks a dramatic rise in support for gay marriage over the years - from a low of 32 percent in 2004 to 47 percent three years ago and on to 58 percent now. Rarely do we see this kind of dramatic change in attitudes on a basic social issue. Vast differences among groups remain - for example, support reaches 81 percent of young adults vs. 44 percent of seniors, and it's 72 percent among Democrats and 62 percent among independents vs. 34 percent among Republicans. Nonetheless support for gay marriage has risen by substantial margins across groups, including those most resistant to the idea overall - Republicans (+18 points in support since 2004), conservatives (+23) and evangelical white Protestants (+24). We also find that 62 percent of Americans now see homosexuality as a way people are, rather than a way they choose to be - up from 49 percent when we first asked this question in 1994.

ABC's ARIANE DE VOGUE: The long line of people sleeping under plastic tarps already snaked around the block at the Supreme Court Monday as hearty individuals hoped to secure seats for today's arguments on marriage equality. But all the Dunkin' Donuts coffee in the world might not be enough to energize the line-holders for the other issue the court will discuss today. When the court agreed to hear the case on California's Proposition 8, it asked both sides to argue not only the merits but a threshold question that could block the court from ever reaching the merits. Proposition 8 is being defended by the original proponents of the ballot initiative - ProtectMarriage.com - because California officials declined to appeal the lower court opinion that struck down the initiative. That's why the Supreme Court asked parties to file briefs on whether ProtectMarriage.com has the "standing" - or the legal right - to be in court. If the court finds that that the proponents of Prop 8 do not have the legal right to be in court, most experts believe that the 9th Circuit Court of Appeals ruling would be vacated and that the District Court decision that struck down Prop 8 on broad grounds would stand. Although legal experts are divided about the scope of the district court's injunction, most experts believe gay marriages would resume in California, although the timing is unclear. http://abcn.ws/X9kDeR

ABC's Z. BYRON WOLF: Unlike with Obamacare, which literally splits the country, the Supreme Court is hearing arguments on something that a fast-growing number of Americans think should be legal (never mind the fact that Prop. 8 was passed by a majority of Californians in 2008). But the country seems on the edge of a tipping point recently. Justices are deciding not whether to make Americans do something - as they did with Obamacare. They're deciding if Americans excluded from something can be included. Who is to say if it is their job to bestow the right of marriage - a union rooted in religion but with legal ramifications - on same-sex couples? The argument against is that states should decide who is and who isn't able to marry within their borders. But with the states increasingly split on the issue, the court has the opportunity to rule on something that increasingly splits the country into two categories: Marriage ban states and right-to-marry states.

ABC's MICHAEL FALCONE: No doubt about it, New Jersey voters are fond of Chris Christie. But here's the catch: They want to keep him in the statehouse rather than send him to the White House. According to the results of a new Quinnipiac University poll out this morning, 70 percent of registered voters in New Jersey approve of the job the governor is doing compared to just 23 percent who do not. Moreover, the poll shows him crushing his likely Democratic rival in this year's governor's race, State Sen. Barbara Buono, 60 percent to 25 percent. And regardless of his opponent, voters by a 66 to 25 percent margin say Christie deserves re-election. However, here's where things get complicated. Forty-one percent of New Jersey voters say that Christie would make a good president, but another 44 percent said "no" he would not. And they are almost evenly divided on whether he should run in 2016 rather than finish out his term as governor should he win in November.

WHAT WE'RE WATCHING

HARRISON FORD'S LEADING ROLE IN SAVING THE PLANET. From Indiana Jones to Han Solo, Harrison Ford has played his share of heroic roles over the course of his career. Now, the actor is back with another hero's mission: saving the planet. But this time, it's not a movie-but real life. Ford, who sits on the executive board of the nature advocacy group Conservation International, has become an outspoken proponent of conservancy, insisting that he is not just a "a poster boy" for the cause. "Nature doesn't need people, people need nature, the nature would survive the extinction of the human being and go on just fine, but human culture, human beings cannot survive without nature," Ford told "On the Radar's" Martha Raddatz. "We have to understand in value what the services of nature are so that we can understand that degrading them is an irreplaceable resource that no amount of money or human ingenuity can replace." Also in this episode, Peter Seligmann, president of Conservation International, makes the case that protecting natural resources is essential to providing for the world's growing population. And Richard Haas, the president of the Council on Foreign Relations, says an unstable natural environment is a major international security concern. WATCH: http://yhoo.it/11Gtq9A

BUZZ

RED STATE DEMOCRAT TIM JOHNSON JOINS SENATE EXODUS. Sen. Tim Johnson, D-S.D., is set to announce Tuesday that he will not seek re-election, according to two sources, becoming the seventh incumbent to retire after next year, ABC's JEFF ZELENY reports. Johnson, who battled back after suffering a life-threatening brain hemorrhage in 2006, is chairman of the Banking, Housing and Urban Affairs Committee. But in political terms he is something else: A Democrat serving in a deeply conservative state. His retirement gives Republicans another open seat to pursue in their quest to win control of the Senate. He has scheduled an announcement at 3 p.m. today at the University of South Dakota in Vermillion, his alma mater, to disclose his plans. One Democratic source in South Dakota and another official in Washington, both of whom spoke on condition of anonymity to avoid upstaging the official announcement, confirmed the decision. Johnson, 66, has spent the past 28 years in Washington. He served three terms in the Senate and five terms in the House. He has faced some of the Senate's most competitive fights for re-election and is one of the last Democratic senators to represent the Midwestern Plains, where Republicans have steadily been picking up seats. http://abcn.ws/X8wZUr

PROP. 8 CASE PLAINTIFFS DECLARE 'HOPE' FOR 'FINAL CHAPTER.' Hand in hand, two same-sex couples, plaintiffs in today's Supreme Court Proposition 8 case, departed for the Supreme Court this morning and declared hope that today will be the "the final chapter in a long four years," ABC's SUNLEN MILLER reports. "Today I am hopeful," Kris Perry of Hollingsworth v. Perry with her girlfriend by her side said outside a row house in Northwest Washington, DC, "I am hopeful we will finally feel the equality and the inclusion that will come with the reversal of Proposition 8." Along with Kris Perry, the other plaintiff, Paul Katami also stood with his boyfriend and said he has "faith in our county's judicial system." "Like all Americans we believe that the institution of marriage helps to build stable and strong families, and that is why we are here," Katami said with his boyfriend by his side, "Jeff and I long to be married and start a family of our own. And have the equal rights that are guaranteed to all Americans that are in loving and committed relationships."

MEET MORE OF THE PEOPLE WITH THE MOST AT STAKE: With lawyers and justices taking the stage now, it's easy to forget that everyday people have a stake in this fight. Here are the stories of some of the people who hope the court will decide in their favor from ABC'S SARAH PARNASS: http://abcn.ws/10aofbA

WHITE HOUSE PUSHES FOR ASSAULT WEAPONS BAN VOTE, BUT SAYS NO NATIONAL GUN REGISTRY NEEDED. The Obama Administration says despite Senate Majority Leader Harry Reid's decision to exclude the assault weapons ban from comprehensive gun control for lack of votes, the President still supports it and urges the Senate to vote publicly on the proposal to eliminate "military-style weapons" from American streets as an amendment, reports ABC'S JIM AVILA AND MARY BRUCE. Speaking in yesterday's White House Briefing, Deputy Press Secretary Josh Earnest said "it will be a question for all 100 members of the Senate to ask themselves about whether or not they think that voting for and supporting an assault weapons ban would actually do something to reduce gun violence in communities all across the country. So, we're going to have that - we're going to have that debate." Earnest also made clear that despite NRA fears, the President does not support a National Registry of legal gun owners. "That is not something that the president has supported", said Earnest. He said the White House has no intention of taking away guns from law-abiding citizens. "What we want to make sure is that we're keeping guns out of the hands of people who shouldn't have them, but without interfering with the ability of law-abiding citizens to get their guns that they would like to buy."

MARCO RUBIO TALKS MARCH MADNESS WITH ABC NEWS. For college basketball fans, late March is the best time of the year. Rubio joined in the Madness, releasing a bracket through his political organization, Reclaim America PAC. Unlike President Obama, who picked mostly favorites, Rubio's bracket was filled with upsets. But don't expect him to root for this year's Cinderella. "I think Florida Gulf Coast has ruined everyone's bracket," Rubio told ABC's CHRIS GOOD, joking that he should have picked the Eagles' upsets, given that he's the senator from Florida and all. He's pulling for his undergraduate alma mater-the University of Florida (3)-when the two teams square off this Friday. "They're fully capable of winning that game," he said of Florida Gulf Coast's matchup with his Gators. "That's a game where I can't lose, right? Whoever wins, wins. I want Florida to win, but it's hard to root against those guys from Gulf Coast 'cause of what they're doing. I would say that the only think I would tell you is that the Florida teams are more aware of them than everybody else," Rubio said. http://abcn.ws/10dAKob

WHAT WE'RE READING

" NOW IN DEFENSE OF GAY MARRIAGE, BILL CLINTON," by The New York Times' Peter Baker. "He had just flown across the country after an exhausting campaign day in Oregon and South Dakota, landing at the White House after dark. But President Bill Clinton still had more business before bed. He picked up a pen and scrawled out his name, turning a bill into law. It was 10 minutes before 1 a.m. on Saturday, Sept. 21, 1996, and there were no cameras, no ceremony. The witching-hour timing bespoke both political calculation and personal angst. With his signature, federal law now defined marriage as the union of a man and woman. Mr. Clinton considered it a gay-baiting measure, but was unwilling to risk re-election by vetoing it. For nearly 17 years since, that middle-of-the-night moment has haunted Mr. Clinton, the source of tension with friends, advisers and gay rights supporters. He tried to explain, defend and justify. He asked for understanding. Then he inched away from it bit by bit. Finally this month, he disavowed the Defense of Marriage Act entirely, urging that the law be overturned by the Supreme Court, which takes up the matter on Wednesday on the second of two days of arguments devoted to same-sex marriage issues." http://nyti.ms/14kcWnW

WHO'S TWEETING?

@zbyronwolf: Why yes, we are live blogging the #SCOTUS #Gaymarriage arguments. http://abcn.ws/16eVG1Y

@ChadPergram: Sens Rand Paul (R-KY), Mike Lee (R-UT) & Ted Cruz (R-TX) write to ML Harry Reid, vowing to filibuster any bill on gun control.

@AaronBlakeWP: Nearly $5 million in debt, Gingrich launches debt-retirement effort http://wapo.st/WTgnhh

@jamesgoldston: Just announced and we couldn't be more proud- @RobinRoberts to receive the Arthur Ashe Courage award this year!

@HuntsmanAbby: Happy birthday to the best dad, and the most authentic politician out there!!! (although I might be biased) @JonHuntsman