本命と対抗馬が競り合ったお陰で、ダークホースの彼が漁夫の利を得たってわけだ。
Since the favorite and his rival were running against each other a dark horse like him was able to pull out in front.
あのダークホースは予備選挙の前は、劣勢でした。
The dark horse candidate was losing in the polls before the primary.
