Article needs some edits.
“High Water Mark” = everything is downhill from here.
As in “Gettysburg was the high water mark of the Confederacy”
I think the author meant to say “largest victory so far...”
+++++
The Kavanaugh confirmation is the high water mark of the conservative resurgence in America
That was PRECISELY my first reaction, too. I figure the author is not adroit at word usage or was very hasty when he wrote this article.
“largest victory so far...” would have been good.
“High water mark so far...” would have been acceptable.
Yes, “high water mark” denotes the absolute maximum level of water in a flood and, as you say, it’s all downhill from there. Very poor word smithing.
I think the title of the piece itself is more accurate. It is in the way "the end of the beginning"; i.e., the end of the lib/commies stomping Republicans.