Writing at RealClearPolitics, the delightfully named Sean Trende reformulates the rule and carries it back a century:
"There are only seven sitting presidents who have ever received less than 60 percent of the vote in any primary: Taft in '12; Coolidge, '24; Hoover, '32; LBJ, '68; Ford '76; Carter, '80; and Bush '92. All of these presidents, with the exception of Coolidge, were not re-elected."
One of Coolidge's challengers, Robert LaFollette, ran a third-party challenge. He ended up with 16.5% of the nationwide popular vote and carried his home state, Wisconsin....
... 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.OBAMA GOT LESS THAN 60% IN 4 PRIMARY CONTESTS - AND NONE WERE AGAINST A CANDIDATE OF ANY RENOWN.
ERGO: UNLESS SOMEONE MOUNTS A THIRD PARTY RUN AS CREDIBLY AS LAFOLLETTE OR PEROT, THEN OBAMA IS GOING TO DEMOLISHED BY ROMNEY.