This metaphor from the Mark Belling radio show illustrates how media bias works in action.
Say there’s a guy who lives down the street from you and you hear a rumor that he is cheating on his wife. If he is someone you don’t like you might spread the rumor. If he is a friend and you think he’s a good guy you probably won’t repeat the rumor.
That’s the sort of natural bias we all have. We indulge ourselves in it more or less according to how strongly we feel about the target of the rumor.
The media believes that Democrats should rule because in their minds a Democrat majority is the natural order of things. They detest Republicans, especially those who have the temerity to win elections.
Therefor, when the media hears something bad about Democrats they simply do not believe it. If they hear something derogatory of Republicans they believe it, no matter how improbable it might be. They will definitely report it and probably even add a few scurrilous comments.
That’s media bias and that’s how it works