Here are the changes in the two parties in the last century:

1. FDR made concessions to the working class in the Great Depression to stave off a communist revolution and protect the status quo. Yet this earned the Democratic Party almost unbroken control over Congress until the 1990s;

2. In the wake of desegregation and the civil rights movement, all the slaver-wannabe Southern Democrats (ie the Dixiecrats) left the party and became Republicans. This was an intentional effort by the Republican Party called the Southern Strategy;

3. Despite 60 years of control over Congress and the loyalty of the working class that went back to FDR, Bill Clinton decided "no, we need to be Republican Lite". This was the so-called Third Way neoliberlism. This is the era when we got the 1994 crime bill, welfare "reform" and NAFTA; and

4. After 40+ years of the destruction of real wages, working people were finally seeking a change in the 2010s. White working class people, who were once loyal Democratic voters, went to Donald Trump. I still believe to this day many of these people would've voted for Bernie Sanders had he been the candidate over Hilary "more of the same" Clinton.

But there's no flip here. The Republican Party just got more insane, overt and racist and the same donors that back Republicans successfully turned the Democrats into the controlled opposition party.