It should be broken into two sentences

Original: "No union victory is bigger than the first win in the United States at Amazon, which many union leaders regard as an existential threat to labor standards across the economy because it touches so many industries and frequently dominates them."

Sentence 1: No union victory is bigger than the first win in the United States at Amazon.

Sentence 2: Many union leaders regard Amazon as an existential threat to labor standards across the economy because it touches so many industries and frequently dominates them.

https://academicguides.waldenu.edu/writingcenter/grammar/run...