It was not a great poll for Trump.
Since the last time the Des Moines Register and Bloomberg polled in the state, in October, Trump's actually gone up two points. But, as then, he trails someone else by a significant margin. The someone else in October was Ben Carson. This time, it's Ted Cruz. We've seen this at the national level -- Carson sinking and Cruz gaining. Now we see it in Iowa.
Notice that Carson's favorability sank as Cruz's rose. On the national stage, Carson's stumble has been tied to his perceived weakness on foreign policy. In Iowa, he's similarly seen as weak on the subject. Only 5 percent of voters think Carson is the best bet to tackle terrorism (compared to 35 percent for the guy with the best numbers in that regard: Trump).
Cruz's surge past Carson is a recent (read: post-Paris attacks) phenomenon. For the first time, Cruz has taken the lead in Iowa in Real Clear Politics's polling average in the state.
The Bloomberg/Register polling tells a similar tale: Trump staying relatively flat since August, while Carson and then Cruz surge past him.
There are two other nuggets in the poll which are good news for Cruz. The first is that he, like Trump, has a more committed base of support. We noted after the October poll that Carson's support was much softer than Trump's, and, sure enough, a lot of it eroded away. The base of support enjoyed by both Trump and Cruz is much more solid.
But Cruz has more room to grow. Thirty percent of Iowans would never vote for Donald Trump, nearly twice the number that say that for Cruz.
Far more, though, say it about Jeb Bush. In the last three Bloomberg/Register polls, the outsider candidates (Trump, Carson, Cruz and Fiorina -- who has about vanished by this point) have gone from eating up 54 percent of the vote to 59 percent to, now, 66 percent. Two-thirds of the vote.
The Iowa caucuses are in 50 days.
Update: Trump decided to tweet.