I dont follow the daily polls like a fever chart (I have a life, unlike Chris Matthews and Frank Luntz), so I dont know exactly when Deans progress stalled and started going into reverse. It is tempting to date it to Al Gores endorsement--is there any more certain kiss of death than to have Gore on your side?
But if you pick apart the polls numbers and look carefully, Ill bet one Dean gaffe stands about from all the others and can be seen as his turning point: his remark that the capture of Saddam made the U.S. no safer. Even most liberals are smart enough to know this is stupid. It took a while for this to percolate, but in retrospect it looks equivalent to 1968s early GOP front-runner George Romney, whose campaign imploded when he made his famous remark about being "brainwashed" over the Vietnam War.
Another time-honored lesson is reaffirmed. Dean is an undisciplined candidate. Undisciplined candidates almost always lose (Hart, Buchanan, Kemp, Eugene McCarthy), where as the disciplined candidates (Carter, Reagan, Clinton, GW Bush) usually make it.