Republicans get in office and implement policies which accelerate wealth inequality and attack workers' rights and minority rights, with a special animosity towards black people.
The Middle American cac power structure laps this up due to shortsightedness and prejudice "the economy is strong right now and those people are getting put in their place
"
Republican incompetence and cruelty gets too blatant to ignore even for these types and they get voted out in the aftermath of an economic crash.
Democrats get in office and implement policies which decelerate wealth inequality and protect black and minority rights, but not convincingly enough due to big business/special interest lobbying and because their control of the three branches isnt complete enough.
Republican propaganda plays Middle America like a fiddle "They making things worse! Plus they're godless/woke/reverse racist!". Dems struggle to combat this due to inferior political messaging and get voted out by an electoral system which advantages Repubs. Repeat.
After over 40 years of this the middle class is now on its last legs, but the pattern shows no signs of weakening