I find this London language school a great source of information on idioms in their ezone.
From this I would suggest A dark horse. Meaning: Someone who surprises you by doing something out of character, or someone who becomes popular despite being previously little known.
If there was 'a dark horse' in the election then the results could turn out to be a surprise. MoreMore on the definition and origin here a dark horse.
You can also see on that link some examples at the bottom of the page which includes one smilliarsimilar to your question. I qoutequote it here : I can't say which team will win - there are two or three dark horses in the tournament.
I can't say which team will win - there are two or three dark horses in the tournament.