The Internet rumor is that he also voted for Carter in 1980 and Mondale in 1984, but he hasn't said so.
In the 70s and 80s there were still old school Democrats in elected office who privately voted Republican in the top race.
I can forgive someone for voting for Carter in ‘76. But I cannot forgive them for voting for Carter again in ‘80. And there is no way I can forgive anyone for voting for Mondale in ‘84.