An August 2007 Washington Post article, A Series of Fortunate Events describes it as follows:
On Tuesday they wanted a keynote speaker in the tradition of the great keynoters of the past: Barbara Jordan, Mario Cuomo, Ann Richards, “people who inspired hope,” as Donna Brazile puts it, “and not only inspired hope, but laid a framework for the party.”
There were a number of criteria as planners began proposing candidates. Youth was desirable, and freshness, and diversity. “We were trying to think creatively of the next generation of leaders,” says one campaign official. They came up with a list of Democratic governors that included Mark Warner of Virginia, Bill Richardson of New Mexico and Tom Vilsack of Iowa: solid choices, but a list that, as the official put it, “didn’t get us where we wanted to go.” Jennifer Granholm, the photogenic new governor of Michigan, was also on the list. And Kerry campaign manager Mary Beth Cahill, who had read some of the coverage following Obama’s primary victory, proposed Obama.
It was an appealing idea. Obama was known to be a speaker who could get a crowd going. He was a Midwesterner from a major industrial state, providing a demographic complement to Southerner Edwards and Northerner Kerry. But these things were also true of Granholm.
. . . .
According to an official involved, the decision came down to the fact that Obama, unlike Granholm, was still trying to win an election. Just a few weeks before the convention, it would emerge that Obama’s opponent, Jack Ryan, had tried to talk his wife at the time into performing public acts at a sex club. Ryan would eventually withdraw, and there was talk that some marquee Republican, possibly former Chicago Bears coach Mike Ditka, would enter the race. The balance in the Senate was 51-48 in favor of Republicans. “We needed his Senate seat,” says the official. So Obama it was.