For the second time in two days, a poll shows Ben Carson with a substantial lead over Donald Trump in the first-in-the-nation Iowa caucuses. A substantial lead: The Bloomberg/Des Moines Register poll has Carson up by nine points -- slightly bigger than Thursday's Quinnipiac University poll showing Carson up by eight.

Thanks to those two polls, the Real Clear Politics polling average shows a giant Carson spike in the state.

Comparing the Quinnipiac poll with one the school conducted in September, you can see how big Trump's drop has been across demographic groups. (It's graphed at the bottom of this article.) Down 11 with tea party members. Down 11 with those between 35 and 49 years old. Down 16 points with those earning between $30,000 and $50,000 a year.

The good news for Trump is that his support is stronger than Carson's. For most candidates, about 22 percent of supporters say their minds are made up and they won't change. For Carson, it's at 15 percent. For Trump? Nearly a third say they won't change their minds.

The Bloomberg/DMR poll also tried to suss out why people backed Carson. The thing voters found most attractive about him was that he approaches issues with common sense -- 70 percent found that "very attractive." Third most popular? He's argument that Obamacare is the worst thing since slavery.

A little lower on the list: 51 percent of Iowa Republicans find his comments about preventing the Holocaust by arming Europeans Jews "very attractive," too.

The poll also asked what would make people less supportive of candidates. Sixty-three percent of Republicans said that Jeb Bush's advocacy of a path to citizenship for immigrants made him less attractive. Forty-six percent said that Trump's insults of other Republicans made him less attractive. Trump was also the candidate most frequently selected when Iowans were asked who should drop out.

So Trump, for really the first time, has taken a back seat to somebody else. Carson, who's very popular still, is having a moment, but his support is soft, and any common sense/guided-by-faith candidate doesn't have a monopoly on those characteristics in this field.

Rick Santorum (polling at 2 percent in the new poll) will remind us that he was trailing the pack before the Iowa caucus in 2012, then surged ahead to win. Making the moral of this story: We will see.