Actually, I'd cite the sheer success of Obama's campaign as the most solid evidence any of the candidates have of their ability to lead and inspire.
Mind you, don't take that as an indictment of Clinton or McCain's time in the senate - certainly I can't question McCain's experience and likely his expertise, but Obama has built what anyone has to recognize as a pretty spectacular grassroots organization more or less out of scratch, against pretty tough opposition. If that doesn't demonstrate an ability to mobilize supporters, provide leadership and run an organization at the executive level, I don't know what does - it's hardly all there is to being a president, but nobody can slight Obama on that particular qualification, anyhow.