That Iowa poll was the latest from Quinnipiac University, which has been polling in the state regularly. In fact, Quinnipiac polled Iowa in December, too, at which point Hillary Clinton led Bernie Sanders by 11 points. That poll was completed Dec. 13. The poll out Tuesday showed a five-point advantage — for Sanders. That's a 16-point swing over the course of a month. (The new survey was completed Jan. 10.)

It's true that Quinnipiac's polling has been more favorable to Sanders over the past few months than others. Plotting polls against the Real Clear Politics average, Quinnipiac's surveys usually fall under the line of Hillary's lead — meaning it tends to show less of a lead than the current trend. But that's in part because Clinton's generally been trending downward, and the polling average necessarily follows from polls like Quinnipiac's.

This also wasn't the only poll to show Sanders with a lead. A survey from American Research Group this week has him up three points.

So what happened to Clinton? Well, part of it is that her favorability slipped. Among all voters, she dropped seven points in the head-to-head matchup (Sanders gained nine), but the percentage of people viewing her favorably fell from 81 to 74.

Among groups that have preferred Sanders (like men), Sanders's lead increased.

Among groups that have preferred Clinton (like moderates), her lead narrowed — usually in concert with shifts in favorability.

On a number of other measures, Clinton held fairly steady. Sanders did gain ground in one important metric: The percentage of people who think he can beat the Republican in a general election jumped from 57 percent to 68 percent. Clinton still dominates here, though, with 4 in 5 Democrats thinking she'll win in November.

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Trends matter in polling. While it's possible that the December Quinnipiac poll overestimated Clinton's support and this one underestimated it, it's clear that the race in that state is close. (Which it has been before, as you can see in September on the graph above.) One bright spot in the poll is that Clinton's support is more committed to sticking with her (84 percent to 73 percent for Sanders). One more bit of bad news is that her support is stronger among people who've never been through the unusual caucus process before.