Fight a losing battle
Definition 1 of 1
Pro Tip 1/3Persistence Tone
The phrase often suggests continued effort despite the likelihood of failure. 
Even though the team is far behind, they keep fighting a losing battle.
Pro Tip 2/3Negative Outcome
It's used when the situation seems hopeless or the outcome seems inevitable. 
He's fighting a losing battle against the advanced disease.
Pro Tip 3/3Not Literal
The phrase is metaphorical, not about an actual physical battle. 
Trying to clean the house with kids around is like fighting a losing battle.