The first time I met Barack we had coffee together at a shop in downtown Chicago. He was in a small law firm, and I was at the Justice Department's civil rights division in the Clinton Administration. Like many who meet him, I hoped he would one day run for public office. You just want people of his caliber to lead.

When at last he decided to run for the Illinois Senate, he called to ask for my help, and I was eager to give it. "I'll contribute at the max," I pledged. "Deval," he said, "in Illinois there is no max." I said, "Brother, I'm sorry, there has to be a max!"

Barack, 46, has already changed American politics. We often hear about the size of the crowds he attracts, as a measure of the excitement about his candidacy. It's the variety of the crowd that is the real phenomenon: little kids who sit on the floor in front of the podium, and the 101-year-old gentleman who stood up from his wheelchair in Iowa and said, "I'm with him too." Farmers in overalls next to people in business suits. Every race, religion and creed. Every political party and no party at all.

You can feel their excitement about being in Barack's presence—and about being in the presence of one another. They glimpse for a minute what it might be like to find common cause across differences. That's how Barack has changed politics.

Patrick is governor of Massachusetts