Suppose you have a two-party system.
One of these parties enjoys/enforces total party discipline, the other, not: members of the latter party side with their own, or cross the aisle, on individual issues/votes, as conscience or self-interest dictate. Let’s call the completely disciplined party the Partisan Party. The completely undisciplined, the Bipartisan Party (to reflect its principled commitment to always keeping the door open to the higher value of bipartisanship!)
Over time, both parties will push positive proposals/ legislation. Quite obviously, the Bipartisan Party will be at a tactical disadvantage, due to its lax discipline. Less obviously, it will have an ongoing optics problem. All the proposals of the Partisan Party will be bipartisan. That is, a few members of the other party will, predictably, peel off and cross the aisle to stands with the Partisans. None of the proposals of the Bipartisan Party, on the other hand, will ever be bipartisan. No Partisan will ever support a Bipartisan measure. In fact, all proposals of the Bipartisan party will face bipartisan opposition – as a few Bipartisans trudge across the aisle (there are always a few!) to stand with the Partisans. Result: the Partisan party, thanks to its unremitting opposition to bipartisanship, will be able to present itself as the party of bipartisanship, and be able to critique the Bipartisan Party, with considerable force and conviction, as the hypocritically hyperpartisan party of pure partisanship.
Conclusion: two measures of partisanship/bipartisanship that you might think make good heuristic sense – 1) being able to get bipartisan support for your proposals; 2) being opposed to those who can’t get any bipartisan support for their proposals – in fact aren’t good heuristics.
(Obviously it’s misleading to hint that the Democrats have no party discipline whatsoever, but the point still stands if modulated to match actually existing actuality.)