Big caveat right off the bat: PPP is the same polling firm that predicted a Hoffman landslide in NY-23. But (a) they had Mitt’s favorables comparable with Huckabee’s earlier in the year, so it’s not like their data’s historically been screwy, and (b) this isn’t just a one-month snapshot but a trend dating back to July.

What gives?

On our monthly 2012 polls, which began in April, an average of 74% of Republicans have held a favorable opinion of Palin to 66% for Huckabee and 56% for Romney.

The gap hasn’t always been that wide though. On the first three iterations of this survey, from April to June, Palin averaged a 76% favorability number to 67% for Huckabee and 63% for Romney. On the last three runs, from September to November, Palin has averaged 72% to 64% for Huckabee and just 51% for Romney.

He’s at 48 percent this month after having hit 63 percent in June, before the fade began. Even PPP doesn’t have any explanation for it. I’ll give you two possibilities. One: Huck and Palin are in the public eye these days much more than Mitt is, even if it is for the wrong reasons. Romney had better be careful that he doesn’t become an afterthought and end up being seen as a “minor candidate.” Two: With Huck and Palin natural rivals for the religious conservative vote, the perception may be building that Romney’s the RINO in the race by default. He’s always had that rep to some extent, of course, but being the odd man out among the big three only cements it. Although maybe it won’t matter: Mitt’s strategy, I assume, is to position himself as the alternative to whoever emerges from the Huckabee/Palin fracas, counting on the fact that New Hampshire will keep him alive in the early running and the “anyone but Huck/Palin” vote will put him over the top.

Speaking of the fracas, here’s video from last night’s Geraldo of Huck defending the woman of the hour. He and Palin are momentarily neck and neck among Iowa Republicans in terms of their favorables — although among Iowans generally, it’s a very different story. There’s no question, I think, whom Romney would rather face in a two-man race. Click the image to watch.

Update: Early consensus among the commenters is that Mitt’s decline, coinciding as it does with the health-care debate, is attributable mainly to RomneyCare. Probably right. No wonder he’s keeping a low profile these days.