Warren, Kaine or Baldwin seem to be the most likely choices.
Why would the Obama people pick Baldwin or Kaine? Remember, the convention is stage-managed by the campaign team of the presumptive presidential nominee. They're going to want to pick a keynoter who'll lavish praise on the party's presidential nominee and/or hurl grenades at the opposing party's presidential nominee. Baldwin and Kaine are running statewide races in swing states. They're not going to want to hug Obama too tightly. Warren, OTOH, is running in a heavily Democratic state, so hugging Obama isn't a problem.
Good points, as usual, from our mod.
