John Glenn wins the nomination. He was running even with Mondale in some polls in 1983; all it took to lose that was a truly inept campaign.
Give Glenn a better campaign manager, split the liberal vote between Mondale, Cranston, and a more successful Jackson (no Hymietown), and Glenn's the nominee. From there, pick Martha Layne Collins as the veep (since Glenn's shaky enough with women as it is), and run a campaign based on the economic downturn in the first years of Reaganism, as what we would now call a Blue Dog Democrat.
If that happens, it'll be close, at least. Because come on: it's John Glenn. (If it didn't promote near-universal ridicule, you could pick Neil Armstrong as veep)