Tom Harkin was standing behind Howard Dean at that scream speech, clapping and yelling, having a good ole time with Dean and the roomful of supporters. Like Mira Liasson did the next day, Harkin could have been all over television and put a stop to all the ugly insinuation that Howard Dean was crazy, over the top. Liasson said she was in the room, she defended Dean saying it was not as television portrayed it.
Why didn't Harkin step forward and do the right thing?
Well, for starters Harkin had given his own "over the top" speech, turning Wellstone's Memorial Service into a campaign rally, so he wouldn't want to remind people of that. More than that?
I think it's highly likely the Clintons got hold of Harkin and told him to let Dean die on the vine.