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. Does anyone know this expression?

Many thanks in advance!

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?