If you saw the poll Morning Consult released yesterday, you probably noticed the 41-point swing against Virginia Gov. Ralph Northam in his net approval rating. That's not surprising, given both the revelation of the klansman-blackface photo on his yearbook page in medical school in 1984.

You might have overlooked the fact that 50 percent of Virginia Democrats still back the man, and only 25 percent of them disapprove. On its face, that seems like the much more remarkable result.

Unless you are very naive, you shouldn't have to ask yourself why this is. I don't think it would be fair to say that half of Virginia Democrats don't really care about racism, but instead just mouth the words to the party's tune — although I'd wager there are quite a few who do just that.

The better explanation is that if blood is thicker than water, then partisanship is thicker than blood. It's hard to let go of an officeholder — an embodied election victory like Northam — for principle, even when the principle is very important.

[Related: Northam popularity drops 41 points amid blackface episode — but half of Democrats still back him]

Some politicians preach gun control while helping the mob import illegal weapons. Some preach respect for women while groping and harassing them. Some preach abstinence education while cheating on their spouses.

But, those are only the worst cases — the true hypocrites. Most of us habitually commit the much more primitive, venial sin of tribalism. When we hear something bad about a politician, perhaps we react immediately by hoping that it's one of the other guys — ahem — because we don't want to face the choice of either disowning the man or disowning, or at least weakening, the principle. And when one of our own side turns out to be the bad guy, some of us handle it better than others.

In this case, Democrats really can't afford to let Northam's offense slide. No one can. At best we could forgive him, but that's a lot harder after his conflicting accounts of whether that's him in the yearbook photo. And it's not as if Northam can beg forgiveness and say he experienced some kind of conversion after the segregation era. That photo is from the mid-1980s! And it reminds me of the South Park flag. If you were trying, for the sake of argument, to create the worst, most flagrantly racist image you could possibly imagine in order to advance a story plot, you couldn't do much better than this picture without actually actually burning a cross in someone's yard.

Whether Northam is the one dressed as the klansman or the one wearing the thick application of blackface — and no, I don't believe his back-up story that he is neither — this photo cannot be admitted as something acceptable. If it were, then it would become impossible to hold anyone in public life accountable again; not for "insensitive comments" or whatever debatable allegations may arise, but for whole-hog, flagrant, over-the-top racism.

The Democratic officeholders calling on Northam to resign understand this. They also get that he's an anchor around their party's neck in a year when they're hoping to make gains in Virginia's state legislature. But the rank and file are evidently slow to pick up on all this.