Could it be that it's not because they refuse to accept the principle ......but that, they simply don't see the same way?
What is considered the "common good" may not necessarily be the same to another, is it? Of course, conservatives and liberals are from two differing ideologies. They tend to see differently.
Though they may want what they think is for the best interest of the people...... what they see as, or how they get their "common good" may not necessarily be the same. Like, security.
Just look at North Korea, as an example - both parties want a NK that doesn't present a nuclear threat. That's a common ground they share. It's the common good.
Well, we saw what method got the NK situation to this point. It's not the kind of method used by previous government(s) - but this differing method seems to be achieving the common good they both (Dems and Reps) want. Release of prisoners is an unprecedented event, a show of good will.
If all works well at the summit with NK (de-nuclearized NK), Dems and Reps will have the common good.... which is, their common ground.