The problem here is people who are working class (a class lower than their parents) but somehow think they are middle class. You've been demoted but no one told you. Sorry.
> whose incomes fall in the middle 60% of American incomes, which is what Brookings defines as the middle class
So we have a conflict of definitions: your socioeconomic definition, and this purely statistical definition. Evidently, many people somewhere got the idea that these two should roughly correspond to each other.
There are also malign incentives around Cantillon effects and central planning generally.
An incumbent will celebrate economic performance under his reign. He can freely cherry pick flattering statistics and his supporters will repeat the same. The state of the economy is immaterial.
Opposition politicians will claim that the incumbent's central planning policies are to blame. When in office the roles reverse. Even if the opposition gives lip service to laissez-faire policies, special circumstances requiring central planning always seem to emerge. Opponents of laissez-faire cite this behavior as proof of debunking.
Regardless of who is in charge, the Fed chair will pick a number and price fix the rate of interest. Partisans squabble. Court economists rationalize. Somewhere beyond this charade economic production happens.
A big part of this is that we seem to have (in the US) geared our system to prioritize the interests of the wealthy at the expense of everyone else.