Partisanship is only objectionable when it becomes an obstacle to the truth. That is, when one becomes a closed-minded partisan hack, concerned with promoting the party line by any (rhetorical) means, regardless of the true merits of the case. The hack treats reasons as mere ammunition, not principled considerations that they'd recognize as having equal weight even if the party roles had been reversed.
Anyone who cares about truth will despise partisan hackery and bias. This much is generally acknowledged. What is less widely recognized is that bipartisanship or 'in-betweenism' is yet another 'tribe' or uncritical ideology. Just as there are blind, uncritical partisans, so there are blind, uncritical fence-sitters. There's nothing inherently better about adopting an 'in-between' position (or suspending judgment altogether). What matters is ensuring that (i) you end up in the position that is best supported by reasons, whichever position that may be; and (ii) you remain open to any new reasons or evidence that may come your way in future.
We should take care to distinguish:
(1) The first-order content of one's present political beliefs/preferences.
(2) Various quantitative dimensions internal to the first-order state, e.g. strength of credence (current confidence level), and weight of preference (or believed importance).
(3) The external 'robustness' of one's current state, i.e. a measure of stability, or how likely one of the above factors is to change in the near future.
(4) One's dispositional commitment to retaining an open mind, i.e. being receptive to reason, and willing to revise one's current beliefs/preferences in light of new reasons/evidence (should such arise).
I've previously noted the unfortunate tendency to conflate strength of credence (#2) with closed-minded 'dogmatism' (#4). These are two very different things, and should not be confused. One is a first-order judgment about the weight of evidence, and the other is a higher-order commitment to revisit this judgment if new evidence comes along in future.
I think similar confusions may be found in common talk about 'bipartisan' politicians. We should take care to distinguish (i) a higher-order commitment to cooperating with whoever is right (no matter their party), from (ii) a first-order commitment to "in-betweenism" or compromising with the other party (no matter whether they're right). Obviously there is nothing particularly laudable about the latter. It is not intrinsically better to "reach across the aisle" than to reach in the opposite direction; it all depends on which potential collaborators could better help you pass good legislation. So a mere disposition towards the middle is no sign of civic virtue. What matters is one's being receptive to cooperating with sane and competent people regardless of their party.
So it is really very stupid when conservatives object that Obama must not be capable of 'bipartisanship' because he is a left-leaning liberal. They are hankering after some worthless notion of centrism (in-betweenism), when what really matters is being bipartisan in the 'good' sense that Hilzoy documents:
Obama tries to find people, both Democrats and Republicans, who actually care about a particular issue enough to try to get the policy right, and then he works with them. This does not involve compromising on principle. It does, however, involve preferring getting legislation passed to having a spectacular battle.
In sum: we shouldn't let party affiliations muddle our thinking. One form of failure here is to uncritically favour one's own party. Another form of failure is to uncritically favour some 'in-between' bipartisan compromise. In both cases, one is irrationally adopting a position for reasons to do with party affiliations rather than assessing the issues on their merits. The reasonable agent assesses matters on their merits, and if they happen to end up agreeing more with one party than another, so be it. It'd be daft to adopt an unwarranted view just so as to appear independent. That's like the "non-conformist" who slavishly adopts the most unpopular fashions. If he were truly independent of the crowd, he wouldn't care whether they end up in the same place.