Steve Hildebrand is a Democratic political strategist based out of Sioux Falls, South Dakota. He was the deputy national campaign director of Barack Obama's 2008 presidential campaign... (wikipedia)
Many will speculate that this signifies that Tom is thinking about running for president, when in fact, that is not what this invitation is all about.
Harkin's good with organized labor, teachers, seniors and environmentalists. And when he endorses, he doesn't just endorse, he actively works for you.
It certainly raises speculation by folks about his future, but he remains in a position at this time that he does not intend to run but doesn't rule it out.