State made a fair point: Vilsack's 1998 win was also a pretty major upset.
I'd have to say that Tom Vilsack's defeat of former Iowa Supreme Court justice Mark McCormick in the 1998 Democratic goobernatorial primary (52% to 48%) was huge. McCormick had a 20 point lead in the polls just a couple weeks before the primary.
Couple that victory with Vilsack's come-from-behind-and-ass-slapping defeat of two-time loser Jim Ross Lightfoot in the general election later that year and this particular double-shot multi-orgasm has to qualify as the biggest political turnaround in Iowa that I've ever seen.
I'll try to narrow this down to bullet points:
Overcame double digit deficits in the polls in two different elections.
Won with no coordinated campaign and an Iowa Democratic Party that was bankrupt.
Defeated a Republican with high statewide name recognition due to previous runs for office and a former Iowa Supreme Court justice.
Overcame a failure to gather enough signatures to appear on the primary ballot.
Was only seen as the third-best Democratic prospect in his own state.
Defeated a 30-year incumbent.
Which upset was bigger? I don't know. I think I'll let you decide. Here's the poll:
I'll post something on the results eventually.