It seems to me that there are many election-related expressions borrowed from the realm of horse racing in English.
I'm looking for an expression meaning
a race or competition where it is difficult to predict who the winner will be.
What would be such an expression?
Neck and neck is a common idiom for a very close political race. Less common is a last minute race. Too close to call might suit, or dead heat. If unknown outcome is due to unfamiliarity with the candidate, he is called a dark horse. If the end is in sight but no one knows who is going to win, it's down to the wire.
"It's Chock-full O' Drugs, followed closely by Stalker, with Old Levis fading fast!" —Commentator, The Simpsons
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. More 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 similar to your question. I quote it here:
I can't say which team will win - there are two or three dark horses in the tournament.
All bets are off would suit. See this question on stack exchange also.
According to abouthorseraces.com, The Paris-Turf Lexicon...
Hat - slang meaning the result was so unpredictable one might just as well have pulled it out of a hat.
But personally I'd probably just say...
...where that link discusses the fact that penalty shoot-outs in football are "a lottery" (the outcome is more down to chance than the skill of the winning team).
“[A] pick ‘em [match-up]” and “[a] toss-up” are listed under what Horizon Sports "touts" as its “Glossary of Horse Racing Terms” and since you included “or competition” in the body of your question, those two terms’ respective definitions would seem to describe what you’re after (i.e., “a race or competition where it is difficult to predict who the winner will be”).
For the sake of honesty, however, I’d characterize them more as general sports betting terms and not as being specific to (or arising from, as far as I can tell) the Sport of Kings (Merriam-Webster):
Pick 'em:
An even match-up; a game with no clear favorite.
Toss Up:
Game where the line is close to pick-em.
