Once is an accident.
Twice is a coincidence.
Three times is a pattern.
So the question is, what pattern are we looking at here?
I think LS named it precisely.
In the old days beginning with Martin Van Buren they said, "Northern men of Southern principles."
It was a winning strategy that elected Democrats to power for many decades before 1860.
In 1856 it elected a Northern Doughface from Southern Pennsylvania: James Buchanan.
The strategy only failed when Southern Fire Eaters refused to accept the "Southern principles" of 1860's leading Democrat candidate, Stephen Douglas.
Fire Eaters walked out of their convention and split their party.
Today all three remaining non-Romney candidates have both Northern and Southern roots, and all are highly sympathetic to the South's conservative values.
And if you stop to consider those who've already dropped out, none quite match those criteria.
Well, I think Mitch Daniels and Chris Christie fit that pattern, but technically they didn’t “drop out” as they never ran in the first place.