go to bat for someone

go to bat for someone

or

go in to bat for someone

mainly AMERICAN
If you go to bat for someone, you give them your support or help. She was just fabulous in going to bat for me, in not being judgmental, and helping me work it out. I wasn't ashamed to go in to bat for Matt. I had faith his acting talent would justify it. Note: This expression refers to a baseball player who joins in the game and takes the place of another batter.
See also: bat