After months of campaigning and more than $200 million spent on advertising, the race for supremacy in Iowa is close in both parties. Among Republicans, Donald Trump appears to hold a slim edge over Ted Cruz, a fiery senator from Texas. Hillary Clinton and Bernie Sanders entered Monday in a surprisingly tight Democratic race, reviving memories of the former secretary of state's disappointing showing in Iowa eight years ago.
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Watching Monday: What to look for in the 2016 Iowa caucuses
At last, it's caucus day in Iowa. A few things to watch for on Monday as the nation gets its first glimpse of where the 2016 candidates really stand in the race for the Republican and Democratic presidential nominations:
WHO SHOWS UP TO CAUCUS
All that matters in the presidential race for the next 24 hours is who shows up to debate and discuss their choice for president with friends and neighbors in Iowa's storied caucuses - and historically, only a small fraction bother.
It's why some of the biggest egos on the planet are begging their supporters to just go, please, to the school gyms, libraries and fire stations that host some of the state's 1,681 caucus sites.
Republican candidate Marco Rubio said late Sunday that he'll speak at three caucus sites Monday - an event center, a community shelter and a church - before the Iowans gathered cast their votes. A spokeswoman for Democratic candidate Hillary Clinton said they had 2,000 volunteer get-out-the-vote shifts filled on Saturday alone.
Republican Donald Trump went with straight-up guilt to inspire his legion of fans. "Wouldn't that be terrible if I lost in Iowa, won everywhere else?" he recently told supporters.
If the turnout numbers are on the high side, boosted by young and first-time caucus-goers, a good night could be in store for Trump and Sen. Bernie Sanders, Clinton's main rival. A lower turnout would likely be a boon for Texas Republican Sen. Ted Cruz and Clinton, who lost the Iowa caucuses in 2008 to Barack Obama.
WILL IT SNOW, AND WHEN?
See: Turnout. A snowfall is expected to begin around the same time as the caucuses, and become heavy enough to become a major winter storm soon after people cast their votes. Trump, again, took his signature subtle approach when it came to the challenge posed by the weather: "You're from Iowa, are you afraid of snow?" he asked of his supporters last weekend.
The weather could also affect how soon the candidates can get out of Iowa for their next date with voters: New Hampshire. The National Weather Service forecast called for 6 inches of snow, or more, to start falling across a wide swath of Iowa late Monday night that will affect the Tuesday morning commute. That could include candidates who plan to fly overnight to New Hampshire, which holds the nation's first primary a week from Tuesday.
Some candidates didn't plan to wait. Ohio Gov. John Kasich ditched Iowa over the weekend, heading to New Hampshire for a head start on campaigning for the primary. Other candidates planned to get out of town early, too, including former Florida Gov. Jeb Bush and New Jersey Gov. Chris Christie. They'll watch the results come in from Iowa among their supporters in New Hampshire.
A SMALLER GOP FIELD?
In presidential nominating contests, it's sometimes said that there are three tickets out of Iowa - just win one to keep going. Once the first set of voters get a say, some of the candidates on the margins are likely to take the hint and step aside.
Some could pivot quickly and throw their support behind another candidate. Former Sen. Rick Santorum and former Arkansas Gov. Mike Huckabee, for example, completed the so-called undercard debate for candidates at the bottom of preference polls and then headed straight for Trump's renegade rally a few miles away.
Trump plans to campaign in Arkansas on Wednesday. Could a Huckabee exit and Trump endorsement be in the offing?
THE RESULTS, OF COURSE
The caucuses are party-run affairs, rather than elections run by the state. And that means you'll need to pay close attention to the outcome.
Santorum's finish in 2012 is a case in point. On the night of the caucuses that year, the Iowa Republican Party initially declared Mitt Romney the winner. But Santorum had actually won, although it took a few days for that result to become clear. He went on to win 11 state contests, delaying for a few months Romney's claim on the Republican nomination.
Tallying the outcome in Iowa is also complicated on the Democratic side, where candidates have to have to support of at least 15 percent voters at a caucus site to become "viable." If not, their backers can either choose not to participate or can join another viable candidate's group. And what the candidates are competing for aren't votes, but support, as determined by a formula that takes past statewide and congressional district voting into account.
The ultimate prize is delegates to the parties' national conventions. There are 30 at stake for Republicans and 44 for Democrats in Iowa, and they're awarded proportionally. In a tight Democratic contest, it's even possible for two candidates to split the delegates evenly.
The Associated Press contributed to this report.