He's looking at American party politics and the current disgrace of the Democrat party and making extrapolations that don't seem to me to comport with the big-picture facts. Progressivism is an ever Leftward drift, hence the name "progressive". It trumps party politics. The "pendulum swing" is not accurate, because it always swings a bit further to the progressive side when it swings that way, and a little less to the conservative side when it eventually swings that way.
Once a given liberty is lost to progressivism; once a societal barrier or taboo is crushed by it; once a societal institution is eroded or broken by it; once society is trapped by another entitlement; those things are lost forever, and permanently in the Left's column.
The author's point that the American electorate is awakening to the horror of what they have allowed to happen on their watch is a valid observation. We may even correct that situation this November.
But that will not end progressivism. It may end us - indeed it seeks to do that very thing - but it will not stop "progressing", no matter to what depths of depravity and disfavor the Democrat party falls.