"the politician has his/her party's policies to defend, the journalist already has his/her slant or angle on the story fixed."

I don't think that's a mind set limited to politicians or journalists. People who can avoid falling into it are pretty rare birds, it seems to me.

The important thing isn't to avoid making a judgement about what is the truth about a situation, it is to remain willing to modify that judgement in the face of new evidence.

"The object of opening the mind, as of opening the mouth, is to shut it again on something solid." (Chesterton)