Rhat gave unions their big push in the 1930s was federal legislation allowing them to be the sole bargainer for employees, even for employees who had no wish to join or pay dues. What we do call an organization that is the sole seller? We call it a monopoly. And not the kind of monopoly that some people say Microsoft is or had been. Microsoft always has to compete with other software companies.

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Unions, moreover, have a pretty ugly track record on race relations, which is why two prominent early 20th century black leaders, W.E.B. DuBois and Booker T. Washington, who agreed on little else, agreed that unions were bad for black workers. When people forcibly prevent you from competing and figure out ways to exclude you from working, you don't feel very good about them.

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Our big challenge with unions nowadays is to rein in unions of government workers who are negotiating high wages and high pensions. That is what is wrecking state and local government budgets all over the United States.

Emphasis mine. Via The Role of Unions, David Henderson | EconLog | Library of Economics and Liberty.