Asked if it's hard to believe this day is really here, Nesbitt tells CBS News correspondent Erin Moriarty, "Yeah. Yeah, it is hard to believe. Certainly, I've always believed that Barack has had the capacity to get here. But there still is a certain improbability to it."
Obama's journey began, Nesbitt says, with an upbringing that gave him his unique perspective on the world: first by living with his mother in Indonesia, and later in Hawaii with his maternal grandparents.
"His mother had very high ideals about the world and what it could be. And then his grandmother was very pragmatic and practical, no nonsense," Nesbitt explains. "And the combination of those two traits, I think have made him almost a perfect politician, because he's idealistic enough to have a vision, but practical enough to say, 'Hey, let's work on what we can get done.'"
It was at the elite, private Punahou School in Honolulu that Barack Obama - as one of a handful of black students - got his lesson in identity. "That's where he started recognizing that the world sees him as an African-American male," Nesbitt says.
He found acceptance on the basketball court, where Nesbitt says Obama is very competitive.
After an Ivy League education, Obama considered becoming a writer. But by chance, Jerry Kellman, a community organizer, came across Obama's resume and convinced him to work in public service in Chicago. "I thought there was a lot of fire there. A lot of passion," Kellman remembers.
Asked why he wanted to do it, Kellman tells Moriarty, "Well, he wanted to learn how to make change on the grassroots level."
After Harvard Law School, Obama made more key choices. Instead of taking one of a number of dazzling job offers, he returned to Chicago and public interest law. And he fell in love with Michelle Robinson, another Harvard grad.
"Some people say that he could never have made it to the presidency without Michelle," Moriarty remarks.
"I think he would acknowledge that," Nesbitt agrees.
Nesbitt is a successful entrepreneur. His wife, a doctor, delivered the Obama children. Nesbitt says he first began to gauge Obama's abilities through - of all things - the game of "Scrabble."
"When we played the husbands against the wives, where he basically carried our team," Nesbitt remembers. "And I started going, 'This guy's pretty smart,' you know?"
Nesbitt became a close friend and confidant, and watched as Obama began to make a name for himself in the Illinois State Senate.
But his rapid ascension in Illinois politics hit a wall when Obama ran for a seat in Congress from a predominantly black district and lost.
"The best thing that ever happened to Barack politically was not getting elected to Congress," Kellman says. "He'd never be president if he had not lost that election. If you're running in the city of Chicago, as part you run as an ethnic politician, you don't get elected unless you narrow your message to that one group of people."
"He realized that, I think, it's possible to not run as a African-American politician but just be who I am, you know, 'I can just be who I am,'" Nesbitt says.
That propelled Obama onto the national stage, a U.S Senate seat, and the keynote address at the 2004 Democratic Convention. Before that speech, Obama seemed to sense this was his moment.
"This crowd built up behind us and I said 'Man, you know you're like a rock star.' And he said, 'Yeah, you think it's bad today, wait until tomorrow. My speech is pretty good,'" Nesbitt remembers.
It was. Three years later, Obama was running for president with Nesbitt as his campaign treasurer.
But when Obama's bid for the White House was almost derailed by his own pastor, Nesbitt says his friend made the most crucial decision of his career: to confront the race issue head-on in a speech in Philadelphia.
"The events surrounding Reverend Wright and his sermons and the endless loop of clips, I think Barack felt that he had to break it by giving this speech on race," Nesbitt explains. "I thought this was a pivotal moment, a hurdle that only he could clear."
"That was just the moment when it all, the reality of it all, set in - that America was right, that he was who they hoped he'd be," he adds.
There will be difficult days ahead, says Nesbitt, but he believes the entire Obama family is ready. "I never worry about Sasha and Malia at all. Malia always has the facts," he says.
Nesbitt says she's like her father. "She might, in fact, be better than her father in some ways. We call her 'The Oracle, 'cause she's just has this great perspective on things."
And friends like Marty Nesbitt intend to be close at hand. "Friends and family are really what we all live for. And that's not gonna change just be he's president."