To understand how income in the U.S. was slowly redistributed upward from the middle class one has to look several places, but certainly a major contributor was the 1980s revision of the prior, steeply progressive, marginal income tax rates that existed following WWII. At one point, after the top marginal rate was slashed and the bottom marginal rate raised, the U.S. rates approached a flat tax with very little difference between lowest and highest marginal rates. This was later, in stages, undone to some extent, however the rates remain today much less progressive than they once were. The 1980 experiments with supply-side economics and flattening of the marginal rates marked the beginning in the United States of redistribution of wealth from the middle class to the wealthy. This, however, is by no means the only factor that resulted in wealth redistribution following the "the great compression of the post WWII period.
The GINI coefficient is a commonly used measure of dispersion for the wealth distribution function for a particular country. A value of 0 means everyone's income is identical and 1 means that oneperson has all the income. In countries we think of as quite socialistic the GINI coefficient is in the 20s to mid 30s. Sweden = 0.23 ;Canada and France, 0.32; U.S., South Africa, Iran, China, Russia, Brazil ~ .45
The following chart is for for tax year 2009. If we had a similar chart from the 1950's it would be immediately obvious how much flatter the rate structure is today. Note that between the fourth quintile and the top 1% there is only a 5% difference. The rates are progressive, but only minimally so.
Finally, here is a silly cartoon I turned up from the Huffington Post.
These charts can all be turned up by googling "Wealth Distribution in the U.S."