go (/slide)downhill; be on the decline ❍ 戴笠一死，毛人凤当上局长，严醉就～了。(罗广斌、杨益言《红岩》 102) So after Dai Li died he had gone downhill when Mao Renfeng was made Director of the Bureau of Confidential Information. ❍ 应该提醒欧阳海，不能眼看着这么好的一个同志～。(金敬迈《欧阳海之歌》391) I can’t let a good comrade like Hai slide downhill.
走下坡路
go (or slide)downhill;go from bad to worse;take an unfavorable turn;be on the decline