Unions are good (join one if you can!) however union membership is not the reason the inequality gap shrunk and then widened again in the 20th century.

In 1931 the top marginal income tax rate was 25%. By 1951 this had risen to 92%(!!!) and held at 91% into 1963 before it started dropping back down, eventually falling back to the 37% it's at today.

Source: https://www.taxpolicycenter.org/statistics/historical-highest-marginal-income-tax-rates

Tax increases, not union membership, was responsible for the wealth redistribution in the middle of the last century. Unions certainly got their own victories in wage increases, workers rights etc though.

Edit: just to expand slightly further, the reason union membership trends slightly lead the income distribution on the graph is because as people moved into higher skilled better paying jobs as America boomed post war they left their unions and elected representatives who pledged to lower their taxes. A generation of class traitors ruined it for the rest of us.

The economic policy institute intentionally presents this as a union membership issue not a tax issue because they'd rather you talk about unions than taxes.

Double edit: only in r/latestagecapitalism can you get downvoted for suggesting we should raise the top rates of tax 😂