My dark horse choice is
John Lewis.
I think he has an impeccable resume. He was with MLK Jr. as an early civil rights leader, he helped keep the freedom rides going, he led the SNCC in its self-defined mission of non-violence, and was pushed aside while holding those beliefs.
The SNCC then radicalized and was led by black power advocates Stokely Carmichael and H. Rap Brown in subsequent years.
He is a southerner. Edwards is a lawyer and I think the republicans would love to make the election in part about tort reform and the ills of unchecked jury awards. Clark is a general, and I think the Republicans would love the issue of foreign policy to dominate the election. (Support W and support our troops)
He is a prominent Civil Rights leader. This is an area where the democratic party is seen as having a more favorable position on, a better established history.
He would be the first African-American on a presidential ticket. This would be a Ferraro type nomination, but I think it would re-energize the African American vote and be just desserts for their strength of support for the national democratic tickets.
He would not be leaving a Senate seat and his district seat seems solidly democratic. I say, let's draft John Lewis.