It started with Richard Nixon and his cynical manoevering.
It was reinforced with Ronald Reagan. Remember how he spun the Iranian revolution and the bad economy on his opposition? Remember how he rode the Moral Majority wave?
It was taken up a notch by G. W. Bush and his band of trigger-happy self-serving country club elitists.
No one in between those points tried to roll back the progress. They're just as guilty. It's been a monotonic increasing function towards the current apex (nadir?).
Fear what comes next. If there is a next.
Yeah the democrats are at fault here too. Clinton had eight years to rollback any damage from Nixon and Reagan. Obama had eight years to rollback any damage from them + Bush. Biden had four years to rollback any damage from them + Trump.
One could argue that the courts or congress/senate may not have been favorable during their time, but that wasn't true during the entire combined 20 years they had.
(siddnote - I think "nadir" was the answer to a crossword clue I was stumped on. I'd never heard it before that. And I thought I'd never hear it again. Interesting coincidence!)