Nixon was pretty leftist fiscally...Before that though, you'd have to go Hoover in '28, before that go all the way back to Teddy in 1904.
I am not sure about Hoover 1928. Not so much about Hoover, actually, as about Smith. Smith went right-wing only after 1933. In 1928 he was a NY governor famous for his public infrastructure projects, whose Lt. Governor was FDR. Had he been elected he may well have been fairly lefty (by the standards of the day) on economics. At the least, I am not sure he was identifiably a right-winger on election day.
Nixon 1960 and even 1968 is more plausible. In fact, the most lefty Republican nominated in the 20th century is, probably, Nixon 72. I would say, he was a borderline socialist by then, far to the left of most recent Democratic nominees - it's just that his competition was even more left-wing that year.
