There are a lot of reasons Barack Obama is poised to be the presidential nominee of the Democratic Party; he's smart, tough, inspirational, hopeful, charming, etc. But here's the main reason he's been able to win: his message is consistent.
Hillary Clinton has gone through numerous campaign slogans. She's gone through a metamorphosis in the last year. Obama has not. Look at his campaign slogan: "Change you can believe in". That was his slogan a year ago and it is his slogan today. That says a lot about Obama and his campaign.
The Obama campaign has not been radically pushed off message. Sure they have strayed now and then. Yes, the Jeremiah Wright fiasco through them for a loop but for the most part, they have stuck to the "Hope and Change" mantra and it has worked. Obama has consistently talked about changing Washington. He's consistently talked about a "new way of politics". He's consistently talked about reaching out to both sides of the aisle. It has all played into this narrative that has formed proclaiming Obama as the "change agen.t"
You see, at the end of the day, people want to believe you're a straight shooter. They want to see your authenticity on display. When you start to "reinvent yourself" or become someone that you're not, people see through that. So when Obama consistently stays with the "Change you can believe in theme" it shows stability. It shows consistency. It exudes a feeling of authenticity. In short, it hits the mark.