Iowa Republicans, 115,000 or more them, are expected to express their choice for the GOP nomination tonight. Nearly every candidate in the field has led the public opinion polls at some point in the Iowa campaign. Those polls continue to be volatile. Here are a few thing to watch to get a sense of how thing might be going early on.
Listen to MPR News' live coverage from Iowa.
Washington Post's Fix highlights six counties to watch.
1. Dallas County
Why to watch it: This is the big suburban county in Iowa, and is one of the fastest-growing counties in the United States. It was the closest county in the state's GOP caucuses in 2008, going for Romney by a mere four votes out of nearly 4,000 cast. It also happens to be the only county near Des Moines that Romney won, while Mike Huckabee racked up huge margins in the central part of the state.
What to watch for: Romney needs to expand his margin of victory here and hope that growing population means growing turnout. Particularly if he loses neighboring Polk County (see below), he would love to be able to make up a lot of those votes in Dallas County and then focus on his more traditional bases of support in the eastern and western parts of the state.
2. Dubuque County
Why to watch it: An eastern Iowa county firmly in Romney's wheelhouse, Dubuque is heavily Catholic and pro-life. In fact, it was one of Romney's best counties in the state, giving him 42 percent of the vote despite qualms in other parts of the state about his Mormon religion.
What to watch for: Former Pennsylvania senator Rick Santorum has made much of his Catholic faith and consistently pro-life record. Somehow upending Romney in Dubuque -- or making it close -- would be a very good sign for Santorum.
3. Johnson County
Why to watch it: Johnson is the home of the University of Iowa and, with it, scads of young voters. (A corollary: Story County, which includes Iowa State University). Young people turned out for then-Illinois Sen. Barack Obama in 2008, but will they come back early from winter break to take part in the Republican caucuses?
What to watch for: Given his reliance on young voters, Texas Rep. Ron Paul must do well in Johnson (and Story) if he wants to have a chance statewide. He took 15 percent in Johnson and 12 percent in Story in 2008 and must do much better this time to win. He probably needs to win both to have a chance at victory.
4. Polk County
Why to watch it: No list of counties to watch would be complete without the biggest county. Des Moines-based Polk County will account for upwards of 20 percent of the statewide caucus vote, and has recently been a pretty decisive electorate, giving one candidate a significant margin of victory (i.e. more than 10 percent). It gave then-Texas Gov. George W. Bush a 2,400-vote win out of less than 15,000 votes cast in 2000 and netted Mike Huckabee a near-3,000-vote margin in 2008.
What to watch for: This was one of the few more urban areas where Romney struggled in 2008, taking just 23 percent of the vote. Given the sheer number of votes at stake, he's got to at least make it close. A win here would be a really good sign for him, virtually guaranteeing a victory statewide.
5. Sioux County
Why to watch it: Sioux, located in the far northwestern corner of Iowa, has the highest Republican registration (by percentage) of any county in the state. It's also widely regarded as the home county of Iowa's social conservative movement. In 2008, Huckabee carried it with a massive 53 percent.
What to watch for: Sioux has to be Santorum country today. While it's hard to imagine Santorum matching Huckabee's lofty percentage from four years ago, the higher he can get his number, the better indicator it will be that he has unified social conservatives behind his candidacy.
6. Woodbury County
Why to watch it: This Sioux-City based county in western Iowa is Romney's base. He netted more votes here (500-plus) in 2008 than in any other county, despite the fact that it's just the sixth-biggest county in the state. But Rep. Michele Bachmann and Santorum have both been making a serious play for this part of the state.
What to watch for: Romney has only visited the northwest part of the state a couple times this year, and he's spending his last few days elsewhere in the state. But if his base holds in an area where he hasn't really spent much time and he wins by as much as he did last time (15 percent) that's a very good sign for him.
The New York Times' Michael Shear doesn't look to geography, but offers up this list of what he is watching.
EVANGELICALS: Four years ago, nearly 60 percent of the nearly 120,000 people who attended the Republican caucuses identified themselves as evangelicals. It was one of the largest percentages in recent memory and it helped power Mike Huckabee to a victory here.
A RON PAUL FADE? The big thing that has always been holding back Representative Ron Paul of Texas is the question of whether he is electable in a contest against President Obama in a race that takes place in the broader electorate countrywide.
TURNOUT: In 2008, at the height of the battle between Barack Obama and Hillary Rodham Clinton, Democrats turned out in record numbers in Iowa. More than 220,000 people gathered in caucuses to choose between the two Democrats (and the others on the ballot).
ROMNEY'S MARGIN (EITHER WAY): Future political strategists may use Mitt Romney's 2012 campaign in Iowa as the model for how to set expectations properly.
FOURTH PLACE: All of the focus on the top three finishers makes sense. Terry E. Branstad, the Republican governor of Iowa, says every time he gets the chance that there are "three tickets out of Iowa." The candidates who finish fifth and sixth will face a lot of pressure to drop out, especially if they are way behind at the end of the night.