We progress through the campaign like this: We begin with big-picture national polls, tossing a thermometer into the ocean to get a sense of how things look. As time goes on, we focus more on polling in early states, figuring out where and how candidates are improving or falling behind in the actual individual elections that will decide the nomination. Then, with those elections looming, we look at the minutiae of the campaigns -- how they're running, subsets of state poll data -- exploring the little ecosystems and tidal pools of what's going on.

Which doesn't mean we get things right, it just means that those things take on more significance as voters start putting on their heavy coats and trundling off to make their decisions.

When we took the temperature of the Democratic race a year ago, it read "Hillary Clinton." That was it. No other reading. That has changed nationally and at the state level over the last few months. We can tell the story by looking at the new Selzer & Company poll in Iowa, sponsored by the Des Moines Register and Bloomberg. One year ago, Clinton was up by 51 points over Bernie Sanders, who few people probably expected to be the one putting up any fight. In the poll released Thursday, Selzer has her up two points.

That graph shows a bit of worse news for Clinton than it might at first seem. In December, she led by nine points, meaning her lead has dropped by seven since then. But that's thanks to her losing six points, not Sanders seeing big gains.

So where did Clinton's six points go, if Sanders only went up one point? Not to Martin O'Malley, who stayed flat at 4 percent. It went to "not sure" -- people who are undecided about who to support. The number of people reporting themselves as "not sure" was 2 percent in December. It's 7 percent now.

We've noted before that people rarely just flat-out change their minds. If you like Candidate X, you rarely spontaneously change your mind and back Candidate Y. Instead, you lose confidence in X, and become undecided. Then, you re-decide. In other words, some chunk of that 5 percent is folks who may be at a waystation between Clinton and Sanders but who once backed the former New York senator.

But that's not really at the "tidal pool" level of analysis, which is where we should be looking at this point. The caucuses are in 18 days!

The Des Moines Register broke out an interesting subset of data from the survey. Fully 27 percent of Sanders's support comes from three counties -- Black Hawk, Johnson and Story -- counties that hold 21 percent of the likely electorate according to Selzer, and counties that are home to major universities.

In 2008, Barack Obama garnered 10 percent of his total support from those three counties, which contributed about 8.3 percent of the total vote. The votes in those three counties were just about equal to the margin he needed to edge out Clinton and John Edwards in a close three-way contest in the state. Johnson County, home of the University of Iowa, was Obama's best county in the state.

Sanders's concentrated support is a problem, as the Register's Tony Leys notes. Each precinct elects a certain number of delegates, so it's better to win a lot of precincts by a little than a few precincts by a lot. Leys also points out that the centralization of votes will be worse for Sanders than it was in 2008; back then, schools were on break and students were scattered throughout the state.

That only matters if the race remains fairly close. Which, we should note, both campaigns are happy to have it be at this point since that spurs donors and turnout. There's not much in this poll to suggest any more big swings -- save a huge, effective ad blitz or some baffling stumble. Nearly all Iowans have learned about Bernie Sanders. His favorability is at 89 percent, with only 5 percent saying they are "not sure" how to feel, which is usually an indicator that someone hasn't heard of a candidate. Clinton's favorability is steady at 86 percent. (Two percent of people are not sure how to feel about her.)

Iowa may come down to weather and to where college students are and to how many volunteers show up at campaign headquarters over the next 18 days. It may come down to a little pocket of water left over from the tide.

And that's what makes politics fun.