The endorsement came at a critical time for Obama; he had won the South Carolina primary by almost 30 points over Clinton two days earlier. It had begun to dawn on everyone that he just might be the nominee. Kennedy's endorsement codified that sense, fueling momentum and energy that helped Obama virtually sweep the caucuses that populated the calendar that February and, in so d0ing, build an unassailable delegate lead.
Of course, it's entirely possible that Obama could have — and would have — won without Kennedy's help. After all, Obama won the Iowa caucuses going away in early January — and then the South Carolina primary — without any help from Kennedy or his clan. (Kennedy died in August 2009.) But the symbolic import of the torch-passing from Kennedy to Obama is hard to overestimate in the context of a race in which one of the young Illinois senator's main hurdles was convincing people that the Clintons, the reigning first family of Democratic politics, could possibly be beaten. The Kennedy blessing mattered. Hugely.
Watch Kennedy's full speech endorsing Obama. It's a remarkable piece of political history.