Actually, there's an eighth sitting president who received less than 60% in a primary--in more than one, in fact. That would be Obama in '12, who, as Trende points out, received just 58.4% in Arkansas, 57.9% in Kentucky, 57.1% in Oklahoma and 59.4% in West Virginia. In Kentucky, his main opponent was "Uncommitted," another name for Nobody.
Nobody is challenging Barack Obama in the Democratic primaries this year--and is doing surprisingly well...
If the trend is predictive--admittedly, a big if--Obama is much likelier than not to lose in November.
And cue the face-palm: