It's still a surprising development. Trump's gained a lot, across the board, while most of his competitors have slipped. Cruz is still over-performing with conservatives and tea partiers (meaning that his support among those groups is 11 and seven points higher than his overall support), but Trump gained 11 and 17 points with those groups over the past two weeks. Cruz's support among the groups fell.

So what's going on? This is the same polling firm and the same methodology. But the voter pool is slightly different.

Two weeks ago, the percentage of respondents saying they would "definitely" go out and caucus on Feb. 1 was 59 percent. In this new poll, that dropped to 54 percent, meaning a 10-point swing toward those who would say they will "probably" go to the caucus. Two weeks ago, Trump trailed Cruz by six points among those who would probably vote. Now he leads with that group by 15 — more than his overall lead against Cruz.

But that's risky for him. As we've noted, self-reporting of whether people will get to the polls is not always accurate and tends to depend on past voting behavior more than anything. In the new Fox poll, Trump gets 34 percent of Iowans, but 43 percent of those who will be going to caucus for the first time. Perhaps they will. But people who haven't voted before are a lot less likely to vote than people who vote all the time, for perhaps obvious reasons.

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Again, Trump's gains are across the board, but he's doing much better with a group of voters that seems less likely to vote. He could certainly win Iowa by an 11-point margin, but that depends on his people turning out — and on his having an operation to encourage them to do so (which the New York Times reports he doesn't). In other words, if the election were held tomorrow, the actual results would probably be somewhere in between these two polls, with Trump not doing as well against Cruz as it may appear.