You know what they say: The darkest moment is always just before you get liberally drenched in glitter.

This hasn’t been Newt Gingrich’s week.

The other day I received a mailing that I instantly knew was from the Gingrich 2012 campaign. I could tell because the stationery had a crest of a sizeable man tripping all over himself.

Wednesday morning Newt called to apologize to Paul Ryan for describing his health-care plan as “right-wing social engineering,” a designation that prompted Ryan to shoot back, “With allies like that, who needs the left?” After the apology, he also told everyone not to use his words in ads. “Any ad which quotes what I said on Sunday is a falsehood, and because — I have said publicly that those words were inaccurate and unfortunate.” Generally, flagging words as “inaccurate and unfortunate” tends to increase rather than decrease the likelihood that they will be used in ads, but that is just typical of Gingrich’s campaign so far.

This isn’t just Murphy’s Law. This is Finagle’s Law — anything that can go wrong will go wrong, at the worst possible moment. Nothing like the week when you’ve announced you’re running for president for you to emerge with anywhere from $250,000 to $500,000 of Tiffany’s bills (that must be a lot of a breakfast!) and inopportune denunciations of your allies’ health-care plans.

Conservative columnist Charles Krauthammer already said the campaign was over. When you have to tell Charles Krauthammer that you are pretty sure that your campaign is probably not dead in the water, that’s a sign that your campaign is not vibrantly healthy.

And he just keeps digging. “When I make a mistake, and I’m going to on occasion, I’m going to share with the American people that was a mistake because that way we can have an honest conversation,” Newt said. Yes, that is how we, the American people, like our potential presidents to present themselves — fallible, ready to make mistakes and talk about them afterwards. And that’s the Newt Conundrum. I an analysis of his candidacy, The Post’s Dan Balz reported that the National Republican Congressional Committee legendarily kept a file cabinet of Newt’s Ideas, most of it reserved for Newt’s Bad Ideas. Newt is that guy who throws everything at the wall to see if it will stick. It’s an awkward gift for a presidential candidate.

And this week, there were more literal things being thrown. For anyone was wondering whether things are going well for Newt, there’s a video clip for you.

See Newt Gingrich, once, the third-in-line for the presidency, the engineer of the Contract with America, the Ideas Man of the GOP, sitting at a table somewhere in Minnesota getting drenched with glitter by a 24-year-old yelling, “Feel the rainbow!”

In the movie adaptation of this campaign, this is the scene right before the protagonist snaps.

If you want to know what the proverbial Darkest Moment looks like, seek no further than Glittered Newt. It sticks on his suit. It gets in his hair. Piles of it stays on the table and he signs books around it. After seeing the video, I almost wanted to rush out and make an “It Gets Better” video, directed at presidential hopefuls whose first week out of the gate goes this badly. It’s almost like that scene from “Carrie” — friendlier and more brightly colored, but equally humiliating.

Still, it’s time to buck up.

Look at it this way, Newt! Things can’t possibly get worse. I mean it! When you’re flat on your back, insisting that we travel back in time to a day before you made those comments about the Ryan plan, there’s nowhere to look but up — into the eyes of the man pouring glitter on you as a commentary.

If the world actually ends on May 21, that will be a boon for you compared to what would happen if this campaign went on another week.

But it’s not over until the fat lady sings. So be extremely, extremely careful of fat ladies. Maybe ban them from your rallies.