The taft-hartley act delivered a major blow to labor by:

The correct answer is outlawing "closed" (all-union) shops.

Basically, a closed shop is when you can't become a member of the workforce unless you are a member of a union. They outlawed this practice which meant that even people who didn't belong to unions could become employed. This was big for the unions who started losing support since people didn't want to join them and pay fees and similar things.