Or like Dukakis in 1988 leading by landslide margins in the summer and then totally blowing it. Candidates leading by large margins then blowing it, usually typical for democrats.
Have you heard of a fellow named Thomas Dewey?
There are more democrats than republicans that have done it. I could list a handful of democrats that have done it. Why do you think that they won only five presidential elections of the fourteen elections between 1952 and 2004?
And I can mention that Bill Clinton was running third in 1992, yet he somehow won the election.
The political environment plays the bigger role, not some metaphysical reason that makes Democrats more prone to act stupidly and lose their leads.
If Reagan in 1988 had the same approval ratings as W. in 2008, then Dukakis would have won in a landslide, no matter how much dirt threw at him Lee Atwater or how many gaffes he made during the debates.