Underdog
Etymology
An American coinage of the mid-1800s, most often traced to dogfighting, where the "underdog" was the animal pinned beneath its rival. The origin is not fully documented and remains somewhat disputed. It now names any long-shot contender in sport, business, or politics.
noun · /UN-der-dawg/
The competitor expected to lose, especially one facing a stronger and better-resourced opponent.
What it names
The sympathy-drawing position of the likely loser — and the outsized meaning of their victory.
Example
Every voter loves an underdog, so she ran as the scrappy outsider even while leading in the polls.
Related idea
The underdog is expected to lose, whereas a dark horse is simply unknown, so its strength can’t be judged either way.