From the blog azizonomics – Union Members and Inequality. Below is a very significant graphic
Union membership and minimum wages are usually thought to reduce inequality by helping the distribution of wages. However although stronger unions and an increase in the minimum wage has the potential to reduce inequality, the fact that wages go above the market-clearing level could mean unemployment increases leading to higher inequality.
Strong unions can persuade political parties to engage in more redistributive policies. By getting workers to vote for parties that promise these policies there is a greater chance of them being implemented.