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.
a losing battle
a struggle that is bound to end in failure.losing battle, (to fight) a
A hopeless undertaking that is nevertheless continued. See lost cause.