What really happened in 1980

[Skill and Chance]

So Carter was polling at 65% in January ?

Rally around the flag.

[Skill and Chance]

Using historical polling, I would say that the best rule  is that the challenger has to be leading the polling averages by double digits coming out of his convention to beat an incumbent.  That would give you the correct call for everything from 1952 to the present.  Truman sneaked through in 1948.

Kerry led the polling averages for most of summer 2004, but never by more than 5%.

[Skill and Chance]

Again, I want to emphasize that no one since 1936 has ever beaten an incumbent without a substantial lead during the summer before the election.  Kerry lost despite leading by 5 during the summer (you could claim he only tied among LV, I suppose).  Romney only managed a tie.  So did Mondale in 1984.

For a late swing, something would have to change very dramatically, probably on the scale of 09/2008.  Negative jobs growth or negative Q3 GDP could push Romney ahead.