The only Northern liberal Democratic presidential candidates since 1964 have been Humphrey, McGovern, Mondale, Dukakis and Kerry.

Comparing how they did in Gallup polling in July versus the eventual electoral result:

- Mondale and McGovern suffered swings toward the Republicans of 2% and 4% respectively, which in their cases didn’t matter much because they started and stayed a devastating 20% or so behind

- In the closer elections Humphrey (1968) suffered a July-November swing to the Republicans of 6%, Dukakis (1988) 16%, Kerry (2004) 9.5%.

In each case in the fall the Northern liberal’s support fell off sharply, when the public found out more about them and the policies they supported.

As a Northern liberal (and he’s a very orthodox, rather left of center one at that) this 40-year history suggests that Obama would need to have a solid 10% lead in the polls now just to have an even chance in November, quite apart from any racial considerations (which if they materialize on polling day may well cut both ways).