My personal opinion is Reagan started the decline of the middle class and the push for money going from the bottom to the top and the republicans have carried on that mantra ever since.
We have data on real average
personal income and on real median
personal income and we can use it to get a sense of how the asymmetry in the income distribution has evolved over time.
Now, before I talk about that, I need to make a
very important point almost everyone seems to get wrong. When you compare the income of, say, the bottom 20% in 1980 to that of the bottom 20% in 1990,
you are not comparing the same people in 1980 and in 1990. You are comparing what it means to be relatively poor in 1980 versus what it means to be relatively poor in 1990, but people move in an out of income brackets far more than you probably think they do. If you care about flesh and blood human beings, social mobility is also important. It's one thing to be poor and it is another to always stay poor. Because we also have data on that, we can also try to worry about people being able to improve their own lives.
With this disclaimer, the data on mean and median income can be found here:
Real Median Personal Income in the United States (MEPAINUSA672N) | FRED | St. Louis Fed
You can click on the EDIT graph button. Put the series in percentage change from last year. Add another series: MAPAINUSA672N (real median personal income). Put it also in percentage change from last year. At the bottom, where it says formula, you can write a-b. If anyone is curious, FRED draws its table from data gathered by the US Census Bureau. Here is how the Census defines income:
About
The resulting series is positive when the median grows faster than the mean. When the median grows faster than the mean, the bottom half of the distribution is gaining ground. And the opposite is true when it is negative. Reagan was president from January 20th 1981 to January 20th 1989. To be fair, say that the results of his policies are 1982 to 1989 (we assume his policies had an impact with a year of delay, in other words). There are 5 years out of 8 where the bottom half lost ground, though only 4 of them really mean something (1984 through 1987) as the 1989 point is a -0.07 percentage point gap -- we might as well say median and average income grew at the same pace that year.
So, there is at least a presumption that your charge might be justified. However, if you look at the worst point of the Reagan administration (about -0.6 percentage point gaps), it pales in comparison to what you can see in the 1990s or 2010s. Things were worst from this point of view in the 1970s, got better in the 1980s, got way worse in the 1990s and after the 2008 recession. There is comparable evidence of the same thing happening under the Carter administration. There also are two worst years under Bill Clinton and years that are many times worse under both Georges Bush (Father) and Barack Obama.
It's not clear what is the problem with Reagan specifically. The dates do not match neatly enough.