I got a question for you: The National Labor Union (NLU), founded in 1866, was the first national labor federation in the United States. It was dissolved in 1872.

Based on this passage, in 1866 the first attempt in the united states at a union for workers was called the?
Let's think first:
The relevant sentence in the passage is: The National Labor Union (NLU), founded in 1866, was the first national labor federation in the United States.
Therefore, the answer is The National Labor Union.