Partisanship is loyalty to a person, or a party, over a principle. It sounds silly. Yet often it is proper.
Rational people, confronted with believing their friend or a stranger, will choose their friend, absent other evidence. This partisanship is amply justified; your friend gets the benefit of the doubt by virtue of being your friend. Since this is looked on favorably, it is usually called not partisanship but loyalty.
A classically partisan decision, in politics, is to vote for a candidate of your preferred party, say the Republicans, over one in the opposing party, say the Democrats, with whom you more closely agree. This too can be rational, provided you have a strong enough preference for Republican over Democratic policies. It is matter of calculating how far your principles will likely be advanced by the preferences of the individual candidates against the party discipline to which each will be subjected. One can err in such a calculation, but voting for the Republican is not wrong on its face.
Wrongful partisanship occurs when you allow your emotions about a person, or a group of beliefs a person holds, to blind you to the merits of that person’s particular view. If you despise Hillary Clinton it is difficult not to find reasons to oppose whatever she supports. On the rare occasions that she is right you will be wrong, out of foolishness and obstinacy. I have a woolly-headed lefty friend who enjoys arguing politics with me despite the fact that he is almost always wrong. He is in thrall to so many fundamental errors that it is nearly impossible for him to see the truth on any issue. I therefore, as a default, if I haven’t already formed a view, oppose whatever he opines. Yet he has been right, like a stopped clock, a few times over the years, and I have been slow to come around every time in my assurance that he couldn’t be right, he’s never right. I am persuaded by this experience that wrongful partisanship is nearly always, at bottom, not partisanship for, but partisanship against.