A political candidate who is not well known but could win unexpectedly WordNet 3.0 & Open English WordNet

A dark horse is a previously lesser-known person, team or thing that emerges to prominence in a situation, especially in a competition involving multiple rivals, that in theory is unlikely to succeed but has a fighting chance, unlike the underdog who is expected to lose. Wikipedia

Political candidate who is nominated unexpectedly; or an underdog in other fields who achieved unprecedented success. Wikipedia Disambiguation

Someone who possesses talents or favorable characteristics that are not known or expected by others. Wiktionary