It was happening a bit before Ronald Reagan, unfortunately. It is popular to see Bill Clinton as the start of this because Bill Clinton was always a dirtbag, but the uncomfortable truth is that it was President Jimmy Carter who pushed Congress to slash New Deal programs and initiated deregulation.
It probably really started happening after the 1972 Nixon landslide. It took Reagan, though, to bring together the Nixon dirty tricksters, the (then-would-be) oligarchs, the John Birchers, the evangelicals and the Confederacy.
yeah, the fall of McGovern really is the moment that the Democratic Party decided to turn its back on the left. crazy that the decisive moment happened so long ago! you’re right, what crystallized that shift was the rise of the New Democrats