Thank one political party

This chart shows marginal tax rates in the U.S. for a century.  Marginal tax rate refers to the proportion of taxes that the average person pays from the last dollar they earn.

Don't even look closely.  The data is obvious. 

One party, in control of both houses of Congress from 1932 to 1980 created the entire increase in tax rates.  The other party, starting in 1980, with the ascent of Ronald Reagan to the presidency, stopped the rise in taxes including the Social Security tax.

The table was put together by Robert Barro and Charles Redlick.