losing battle, (to fight) a

a losing battle

A failing or hopeless effort; a situation or activity that is ultimately futile or cannot be won. Most often used in the phrase "fight a losing battle." I'd give up trying to get your brother to agree to this deal if I were you, it looks like a losing battle at this point. You're fighting a losing battle if you think you can convince Sarah to go to college.
See also: battle, losing
Farlex Dictionary of Idioms.

a losing battle

a struggle that is bound to end in failure.
See also: battle, losing
Farlex Partner Idioms Dictionary

losing battle, (to fight) a

A hopeless undertaking that is nevertheless continued. See lost cause.
See also: losing
The Dictionary of Clichés by Christine Ammer
See also:
  • a losing battle
  • losing battle, a
  • a losing game
  • caucus race
  • ye
  • abandon hope, all ye who enter here
  • all hope abandon, ye who enter here
  • rearrange (the) deckchairs on the Titanic
  • rearrange the deckchairs on the Titanic
  • shift (the) deckchairs on the Titanic