@okie,
The answer ought to be simple. Since 1979, the bottom four quintiles . . . and a quintile is 1/5th or 20% . . . have seen no real increase in their salaries.
Last year, I made less than $19,000. I taught two classes at a community college one semester and earned $5,380 or less than the taxes on my house for a year.
My father made $17,000 during the year that he earned the most, which was probably in the 1970s. He retired in 1984 after he had his first heart attack at age 61.
My family of six lived better than I did last year or the year before.
I paid taxes the year before. My return was corrected by the government and the amount I owed was doubled. Unless the government corrects my return, I will pay no taxes this year but to reach that point, I had to earn less than the amount regarded as the minimal for this area: $22,000. (When Michael Harrington wrote The Other America, grad student poor made $2,000/annum . . . today's grad students have to earn more than $20,000 just to survive.) Then I had to use some of my returns from my reverse mortgage to install a pellet stove in my house, which is the only way I was able to not pay taxes . . . or might be . . . we will see what the feds have to say.
I would suggest that the 40% who pay no income tax are not people like me who earn very little but people at the top . . . who would tend to be Republicans. Although the Republicans are angry at both Warren Buffet and Bill Gates because the Repubs find those guys far too liberal.