The last five polls of Iowa, if you include that one conducted last week for Bobby Jindal’s Super PAC: Carson by eight; Carson by nine; Carson by eight; Carson tied with Trump; and now, from Monmouth, Carson by 14.

Which, do you suppose, is the outlier?

Compared to two months ago, Carson is up by 9 points and Rubio is up by 6 points. Trump has dropped by 5 points and Fiorina’s share of the vote has also decreased by 5 points. Carson’s support has gone up among all ideological groups. He now leads Trump by 9 points (31%-22%) among very conservative voters compared to a single point two months ago and by 21 points (39%-18%) among somewhat conservative voters compared to 2 points in August. He also has a 17 point (29%-12%) lead among moderate to liberal voters, which is a reversal since the prior poll when he trailed Trump by 9 points (17%-26%) among this group.

Carson maintains a 36% to 18% lead over Trump among evangelical Christian voters, which is somewhat larger than the 29% to 23% advantage he held two months ago. However, Carson also holds a 28% to 19% edge among non-evangelicals, reversing an 18% to 24% deficit in August. Carson’s 34% to 17% lead over Trump among women is similar to his 30% to 19% advantage two months ago. He now leads among men as well, 31% to 20%, which wipes out the 17% to 27% deficit he had to Trump in the prior poll.

Not only is Carson the first choice of a plurality of Republicans, he’s their second choice too at 19 percent. Trump is the second choice of just 12 percent of voters, trailing both Cruz and Rubio (who are tied at 14 percent). Not only has he slipped in the polls, in other words, but other candidates’ voters aren’t primed to stampede towards him as a fallback if Carson falters.

What’s knocked Trump down in Iowa? The Club for Growth is taking credit, claiming the $1 million it dropped on ads in the state attacking Trump as a phony conservative did the trick. Okay, but … why would those ads be working on moderate Republicans and non-evangelicals? Also, neither Trump nor Carson have seen wild changes in their favorable ratings. Carson’s still sky high at 84/7 while Trump is at 53/38 (versus 52/33 a month ago, a bit of deterioration) yet Carson’s gone from a 10-point deficit among men to an 11-point lead. Maybe his refusal to back down under media pressure on opposing a Muslim candidate for president and championing gun rights as an obstacle to tyranny convinced some Iowa Republicans that he, not Trump, is the real “anti-PC” candidate in the race. Or maybe Trump insulting Carson as “low energy” and sniffing around his religion alienated some voters — although you’d expect to see a steeper drop in Trump’s favorables if that were true. There’s no obvious reason from these numbers why some meaningful number of GOP voters in Iowa would suddenly decide they prefer Carson’s brand of outsiderism to Trump’s, but there’s also really no doubting after five polls that that’s happening. Could be it’s as simple as Iowans reverting to form. Carson is the sort of pious underdog whom Republican caucusgoers have preferred in the last two cycles. No one would be surprised if they went that way again, Trump’s best efforts to the contrary.

Speaking of which, a good point from MSNBC’s Benjy Sarlin:

Right, and that’s only half the equation. Carson’s not only potentially a Trump-killer in New Hampshire, he’s a Cruz-killer too. I’d be surprised if some establishmentarians in the donor class haven’t already quietly cut checks to his Super PACs to make sure he’ll have the money he needs to win that state. Cruz will be more dangerous in South Carolina if he’s coming off a win in Iowa, and South Carolina is where donor-class favorite Marco Rubio is supposed to shine. Trump doesn’t need Iowa as badly as Cruz does, although a humiliating loss there would hurt his image simply per the logic that Donald Trump is never supposed to lose at anything, particularly in humiliating fashion.

Which brings us to one of the mysteries of the campaign to date: Why isn’t the billionaire spending more of his own money on ads to take down the competition? Trump’s answer to that thus far has been that he’s worried about overexposure — how much more Trump can voters take when he’s on cable news 24/7? — and, it seems, a certain pride that he’s managed to build his lead without having to drop the sort of exorbitant sums that Jeb Bush’s PAC is prepared to spend to promote him. On some level, I suspect, he thinks he’s so charismatic and so practiced at the art of the insult that a paid ad couldn’t possibly be more effective in taking down one of his rivals than he himself is during a cable news hit. But that’s goofy. An ad that attacks Carson as dangerously inexperienced could obviously damage him more than Trump babbling about how sleepy he looks. And though it might surprise Trump to hear this, not every voter watches all of his interviews and televised rallies on cable news. (Believe it or not, some voters don’t watch much cable news at all.) That’s why candidates need ads: The frequent rotation across different networks reaches viewers who don’t watch “Hannity,” say. Trump may have begun to believe his own BS about changing the rules of politics, that he’s so singular a talent that he can build a winning movement without running ads or, er, even obtaining a voter file. If New Hampshire starts to get tight, I bet he’ll rethink that.