In 1919 a little known horse named "Upset" defeated arguably the greatest race horse in history named Man o' War in the only loss in that horses entire career.
Since that time in sports when a underdog team defeats a superpower in an unexpected win it is called "pulling an upset" named after the 1919 race mentioned earlier. The term has spread to other things including elections.
The winner of this category is a candidate who won their election when absolutely no one (perhaps even the candidate themselves) expected them to win.
list of winners
2022 - US Rep. Marie Gluesenkamp Pérez (D-WA03)
(We can perhaps go back in time if this is a popular category
