What tactic used by unions involved workers refusing to work until their demands are met?

The tactic used by unions where workers refuse to work until their demands are met is called a strike.

Explanation:

A large union might call a strike in order to leverage better working conditions for union members, like a teachers' strike or air traffic controllers' strike. Organizations might also call a general strike when many union and nonunion people might decide to forego work and other activities in support of a change or to protest an issue. One of the largest historical examples in the United States is the Pullman Strike, which took place in 1894. From May to July of that year some 250,000-factory workers walked off the job at the Pullman Palace Car Company in Chicago to protest long workdays and reduced wages. The American Railway Union joined forces with the strikers and refused to work on or run any trains that were transporting Pullman products.