His first big subject was belief itself. Instead of waging a partisan campaign as Clinton had just done, he vowed to address “not just Democrats, but Republicans and independents who’ve lost trust in their government but want to believe again.”

Then he made a broader attack on the political class, and without mentioning her, threw Clinton in with the decrepit old order. “The same old Washington textbook campaigns just won’t do,” he said, in a now familiar line. He said it was time to “finally tackle problems that George Bush made far worse but that had festered long before George Bush ever took office  the problems that we’ve talked about year after year after year.”

Obama sketched out a different theory of social change than the one Clinton had implied earlier in the evening. Instead of relying on a president who fights for those who feel invisible, Obama, in the climactic passage of his speech, described how change bubbles from the bottom-up: “And because that somebody stood up, a few more stood up. And then a few thousand stood up. And then a few million stood up. And standing up, with courage and clear purpose, they somehow managed to change the world!”

For people raised on Jane Jacobs, who emphasized how a spontaneous dynamic order could emerge from thousands of individual decisions, this is a persuasive way of seeing the world. For young people who have grown up on Facebook, YouTube, open-source software and an array of decentralized networks, this is a compelling theory of how change happens.

Clinton had sounded like a traditional executive, as someone who gathers the experts, forges a policy, fights the opposition, bears the burdens of power, negotiates the deal and, in crisis, makes the decision at 3 o’clock in the morning.

But Obama sounded like a cross between a social activist and a flannel-shirted software C.E.O.  as a nonhierarchical, collaborative leader who can inspire autonomous individuals to cooperate for the sake of common concerns.

Clinton had sounded like Old Politics, but Obama created a vision of New Politics. And the past several months have revolved around the choice he framed there that night. Some people are enthralled by the New Politics, and we see their vapors every day. Others think it is a mirage and a delusion. There’s only one politics, and, tragically, it’s the old kind, filled with conflict and bad choices.

Hillary Clinton has fought on with amazing resilience since then, and Tuesday night may well bring another surprise, but she’s always been the moon to his sun. That night in November, he defined the campaign.