George McGovern voted for Gerald Ford in 1976 (this one still makes no sense to me)
because jimmy peanuts was a scary fundie
This isn't a Presidential nominee, of course, but I feel obliged to say something here about Insane New South Swamp Creature William Donald Schaefer being too much so to even endorse Bubba in '92–claiming to have a strong rapport with HW–and thereafter his performative feuding with his successor as governor, Parris "Ayatollah" Glendening (very unfairly maligned in his time even if his legacy is still rather mixed).
I don’t think it was because Carter was a fundie but more just revenge because Carter had been part of a cadre of Dems working to thwart McGovern’s nomination in 72.
