Idiom that means trying to save something that is beyond saving

"Grasping at straws" is probably the best I can think of but I think it is a weak phrase.

Close

1. A fool's errand
2. Herding cats
3. A Sisyphean task
4. A Pyrrhic victory (if you accomplish the immediate task)
5. To win the battle but lose the war

Imperfect

1. Tilting at windmills, or battling windmills
2. Banging your head on the wall
3. Bailing out a barge with a bucket
4. Biting off more than you can chew (more about the aptitude of the actor than the futility of the task)

Consider 'futile' or '... an exercise in futility.'

"lost cause"

as @Joseph-Neathawk wrote.

or just simply "doomed" (which is just an adjective not an idiom)