> Because it’s clear to me that that’s when journalists believed that their moral obligation was to further the agenda of their respective political parties and not care about the absolute truth.

For that to be harmful, it doesn't even have to be all journalists, just enough of them that people come across that bias often enough to be familiar with it.

I'm certain that many journalists value the truth and fairness over party, but I'm also certain that too many journalists put party/ideology over truth and fairness (in many areas), and the news organizations have become more tolerant of bias.