(something) is a long shot
A "long shot" is something that has a very low chance of succeeding.
A: About the keynote speaker – do you think that we could get the governor to do it?
B: Hmm. Well, it's a long shot, but it's worth a try.
To remember "a long shot", imagine someone who's trying to shoot an arrow at a target. The shooter is very far away from the target, so it's unlikely that the arrow will hit it.