I am really skeptical of the "X had a big lead after the convention, but (s)he did much worse in November, therefore the fault must be with X's terrible campaign" narrative that is so common here. As I wrote in a post some time ago (dealing specifically with 1988, where I acknowledge that Dukakis ran a bad campaign but express doubt that this was why he lost):
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George W. Bush led Al Gore by seventeen points after the GOP convention in 2000.
http://www.gallup.com/poll/2338/maj...-election-primary-season-party-conventio.aspx
That he actually lost the popular vote to Gore may be explained in one of three ways: (1) Far from being a genius, Karl Rove was an idiot, and Bush ran an incredibly bad campaign. (2) Al Gore's campaign, so criticized at the time, was absolutely brilliant. (3) Nobody should take huge leads after a party convention too seriously.
I don't think you'll be surprised to learn that I lean toward (3). Other examples besides 1988 and 2000: Jimmy Carter led Ford by 35(!) points after the Democratic convention in 1976. If you will say "Well, the fact that Carter almost lost shows that he was a poor campaigner, too" then consider this--Ronald Reagan led Carter by 28 points after the GOP convention in 1980.
https://books.google.com/books?id=DzTvCgAAQBAJ&pg=PT115 Yet he ended up winning by "only" ten points. This shows the absurdity of taking such leads seriously. Unless you believe that by a strange coincidence all those candidates with huge post-convention leads were terrible campaigners.
https://www.alternatehistory.com/fo...-first-degree-murderers.411266/#post-14345962