That poll shows Trump in second place nationally, two points behind Ted Cruz. This is the first major poll in which Trump hasn't led the field nationally since October.
NBC and the Journal also polled in January, shortly before the actual voting began. Since then, Trump has fallen 7 percentage points and Cruz has gained 8, with a margin of error of 4.9.
Other polls, like one from Quinnipiac University out on Wednesday morning, show Trump having apparently vacuumed up support from some of the candidates that dropped out of the race. Combined, they had 12 percent of all support in January. In NBC/WSJ's estimation, though, that support apparently went mostly to John Kasich and Cruz.
This is a very, very surprising poll, to the extent that NBC's press release on it notes how much of an outlier it is right now. Perhaps, one of the pollsters writes, it shows that we are "right on top of a shift in the campaign." It also notes that the ideological make-up of the people they asked was much more conservative this time around, favoring Cruz -- but even if the results are weighted to what they saw in January, Trump and Cruz are basically tied.
It's clear that the field has been stirred up a bit since voting started in Iowa on Feb. 1 -- something rendered more dramatic by the relative dearth of polling. But if you compare this poll with the Real Clear Politics polling average, which showed Marco Rubio -- not Cruz -- surging, you can see how far out of line these new numbers are.
(The January NBC/WSJ poll also showed a tighter Trump-Cruz race than the polling average, but not nearly as dramatically.)
Put another way: How can Quinnipiac release a poll showing Trump up by 21 points and NBC/WSJ show him down by 2? One possibility is that "shift" is perhaps pegged to Trump's much-criticized performance in last weekend's debate.
The NBC/WSJ poll is the first national poll conducted entirely after the debate. Quinnipiac's poll was in the field from Feb. 10 to 15 -- straddling the debate on Feb. 13. NBC and the Journal talked to voters from Feb. 14 to 16. Quinnipiac did indeed have Trump at 40 percent in interviews conducted before the debate and 31 percent afterward, but that 31 percent is still above what Trump sees from NBC and the Journal.
Also possible? This poll, for whatever reason, exaggerates a tightening of the race that Trump still leads. Over the short term, Trump is poised to win South Carolina and Nevada with apparent over the next week. If this poll shows how the future looks, though, Ted Cruz may suddenly be a significant hurdle between Trump and the nomination -- just when it was looking like there might not be one.