Christopher Hayes expresses it as well as anyone I’ve read:
The question of who can best build popular support for a progressive governing agenda is related to, but distinct from, the question of electability. Given a certain ceiling on Clinton’s appeal (due largely to years of unhinged attacks from the “vast right-wing conspiracy”), her campaign seems well prepared to run a 50 percent + 1 campaign, a rerun of 2004 but with a state or two switching columns: Florida, maybe, or Ohio. Obama is aiming for something bigger: a landmark sea-change election, with the kind of high favorability and approval ratings that can drive an agenda forward. Why should we think he can do it?
The short answer is that Obama is simply one of the most talented and appealing politicians in recent memory. Perhaps the most. Pollster.com shows a series of polls taken in the Democratic campaign. The graphs plotting national polling numbers as well as those in the first four states show a remarkably consistent pattern. Hillary Clinton starts out with either a modest or, more commonly, a massive lead, owing to her superior name recognition and the popularity of the Clinton brand. As the campaign goes forward Clinton’s support either climbs slowly, plateaus or dips. But as the actual contest approaches, and voters start paying attention, Obama’s support suddenly begins to grow exponentially.