It depends how the terms are defined. As things would have been seen at the time, then you have to go back a long way. 1924 is tempting but I would argue that both major party candidates were conservatives (different types, though), so maybe 1916. But the last time it was clear would be 1904.
Now if we're using contemporary ways of looking at things, that might differ.
This (though I would go with 1924).
Though perhaps 1928 could be argued with retrospect; given what Al Smith tried to do in the 1930s.
