You give CNN way too much credit. This story acts like their bias began with Trump. Ever hear of Candy Crowley? Yes, they were great in the Gulf War, but they were named the Clinton News Network for a reason back in the nineties, and in 1988, they signed off their final post presidential broadcast of Inside Politics without ever mentioning that George Bush had beaten Dukakis. They were as bitter then as they are now.
CNN came on in the spring of 1980, advocating for Ted Kennedy against Jimmy Carter. They had some good journalists like Don Farmer and Chris Curl, but the network was never middle of the road.
Agreed, CNN was recognized as liberal propaganda long before Trump ran.
Look instead to ownership changes. CNN was founded in 1980 by Ted Turner. He sold it in 1996 to Time Warner.