The Democrats are stewing in a caucus mess of their own creation with the sloppiest train wreck in history.

1. with 表示“原因”。

2. in a mess：处在混乱中。caucus 是名词“团体；集体”，放在 mess 之前说明“混乱的性质”。of their own creation 是 a mess 的定语，意为“他们自己制造的混乱”。

所以，in a caucus mess of their own creation，意思是：处在他们自己制造的集团乱局中。