请教一句话的理解

tiatiatia

各位高人帮忙看看这句话的结构：

Moreover, the rationale for Herbert's emphasis on the social and political realities that impressionist painting can be said to communicate rather than on their style is finally undermined by what even Herbert concedes was the failure of impressionist painters to serve as particularly conscientious illustrators of their social milieu.

第二行中的was the failure...的主语是什么啊？和前面的is finally undermined 啥关系呢？

谢谢啦！