Michael Dukakis was a Democrat candidate running for President in 1988. He had previously been criticized as being soft on national defense, so in September of that year he orchestrated a photo op in an M1 Abrams tank meant to toughen up his image.
The public found the image of Dukakis in the oversized safety helmet comical, and the photo op had the opposite of the intended effect of making him look tough. So much so that his opponent, George H.W. Bush used footage of Dukakis' tank ride in his own advertisements.
Dean's campaign was dead and starting to rot before the scream happened. It wasn't the death so much as the obituary.