It wasn't just the scream. At the time, his speech made him seem unhinged and unstable. It gave the impression that he was another maverick like McCain.

While the speech was actually well received by his core supporters, Dean needed to broaden his appeal and project that he's a serious, rational leader fit for the presidency and all of the responsibilities that it entailed.

Instead managed to convey the complete opposite. His aides tried to blame the microphones, argued that the clips were doctored to make him look bad, etc., but in any case voters thought that he was "unpresidential" and he completely tanked in the subsequent primaries.