It depends on what you mean when you say it.
There is a world of difference between 'biased toward' and 'devoted to'.
There is a difference between seeing the opposition as enemies to attack, harrass and destroy and seeing them as people who hold differing views and values but with whom one can come to an understanding.
At any time? Yes, I'm sure many of us can particularly those who are unsure and those who don't care at any given time about a given issue.
I'm mildly surprized that more people don't even try.
Not necessarily.
Does it make others less credible?
Some people derive their credibility from their partisanship and are proud of it.
It depends on what is meant
by calling someone else partisan and the circumstances of the accusation.
Because the word covers a range of meanings, it lends itself to weaselly arguing of semantics - claiming that someone who attacks and harrasses is the same as one who is simply biased.