In seven of the last nine competitive Democratic primaries, the candidate who placed first in the Iowa caucuses went on to become the party's presidential nominee.
Here are six things to watch in the Iowa caucuses:
How many people turn out?
One question that could determine Monday night's outcome is how many Iowa Democrats turn out to vote: Will it top 2008's record of nearly 240,000 caucusgoers, as Iowa Democrats for months had expected? Or will it be somewhere between that record and 2016's more muted turnout of about 171,000?
Sanders' campaign -- which has emphasized finding new voters, particularly Latinos -- believes the higher turnout climbs, the better it is for them.
Biden, meanwhile, could benefit from lower turnout, with his support coming from older, traditional Democrats who never miss caucuses.
But it's not all about the total turnout. Campaigns will closely watch entrance polls to see the age of the electorate. In 2016, for example, 28% of voters were 65 or older. It's a group Biden performs best with, he'd benefit if it increases.
Region matters, too. Warren has concentrated her campaign on the suburbs -- the sorts of areas where voters, especially women, propelled Democrats to victory in the 2018 midterms. If that surge carries over in 2020, it'd be a boon for her hopes.
Buttigieg, meanwhile, has been focused on turning out disaffected Republicans in rural areas. If those Republicans and independents make up a larger-than-usual share of the electorate, it'd be good news for Buttigieg, Biden and Klobuchar.
Which vote counts?
Iowa Democrats are releasing more information about caucus results than usual this year -- and it could cause some confusion, and give more than one campaign reason to declare victory.
Each precinct sends delegates to county conventions, who then send delegates to the state convention. The "state delegate equivalents" -- the number that'll be the focus Monday night -- are the estimated number of delegates candidates have won to the state convention based on their results in each precinct.
But another number that some campaigns are likely to emphasize is the raw vote total.
The state Democratic Party will release the total number of people at each precinct that lined up with each candidate at the start of the caucuses.
Then, after realignment -- the process where those who fall short of 15% are eliminated, with their supporters either choosing a different candidate to back or going home -- the state party will release those raw vote totals as well.
It's possible the same person won't win all those categories. Some candidates could turn in strong performances in urban areas but fall short of 15% in rural areas and fail to pick up any delegates -- or vice versa.
Campaigns are likely to pick and choose the data that's best for them to emphasize.
Battle to win the moderate lane
The three top moderate candidates in the race -- Biden, Buttigieg and Klobuchar -- are all running to win on Monday night.
But they are also running to finish ahead of each other.
The thinking goes like this: With most operatives anticipating a Sanders win on Monday night, the three candidates want to be able to leave the state claiming the title of top challenger to the Vermont senator.
This is because the race after Iowa will likely be viewed after as a contest between one or two progressives and another group of moderates, so finishing atop an ideological cohort could give the top candidates needed momentum in New Hampshire.
"Let's face it. I need to have a good finish here in Iowa," Buttigieg said on Sunday.
Biden, too, has been playing pundit and publicly downplaying the need to be first in Iowa.
"There's a big difference between second and fourth," Biden told CNN in Mount Pleasant. "I think it's going to be really tight no matter how it works out. It's been bunched up. It's going to remain bunched up I think."
He added: "I just think it's a different year in that I think the measure, you all won't do it now and I don't mean it in a bad way, but I think what you're going to have to measure is who can represent every aspect of the Democratic Party."
What happens in the Obama-Trump counties?
Iowa has more counties that voted for Barack Obama in 2012 but backed Donald Trump in 2016 -- and almost every campaign in Iowa has looked to court voters in these states as a sign of their ability to take on the President in 2020.
What happens in these counties -- including who wins and what turnout is -- will speak volumes about each candidate's ability to win in contested counties and whether Democrats' theory that some disaffected Republicans are ready to break with Trump is true.
No candidate has made this pitch more forcefully than Buttigieg, who spent the final weeks in Iowa rallying supporters in many of these areas
"Regardless of how you voted before -- what party, if any -- you belong to you belong here," Buttigieg said this week in Cerro Gordo County, a place that went to Obama in 2008 and 2012 but went for Trump in 2016.
Klobuchar is the only candidate that has visited every so-called pivot county, and her campaign has made her ability to win over rural, conservative voters central to her pitch.
"I see a coalition that is much bigger than what some people see," Klobuchar said on Saturday. "I see fired up Democrats and I see people in rural and suburban, and I want to bring them with us and we really need to bring with us in this general election."
What happens in these pivot counties -- especially Cerro Gordo, Dubuque and Clinton counties -- will be telling, not only for who wins the Iowa caucuses, but how they are positioned going into November.
Bernie's gamble on turnout
Sanders waged his campaign in Iowa with a clear proposition: that higher turnout among working class, young and Latino voters would all but guarantee victory.
If his bet comes in on Monday night, the campaign is confident that it will head to New Hampshire, where Sanders won handily in 2016, with both history and momentum on his side.
Sanders' Iowa team has been organizing in Latino communities and in other parts of the state with populations that don't traditionally tend to make big showings on caucus night for nearly a year. They have organized soccer games and advertised aggressively on Spanish-language radio. They have camped out organizers and volunteers in Casey's stores around the state, in order to meet working class voters where they shop and kibbitz.
Latinos only make up about 7% of the state's population, but the community skews younger -- so, as seen in the polls, likely toward Sanders -- and could potentially provide the boost he needs in what is expected to be an extraordinarily tight final count.
As many Sanders aides are quick to note or ready to concede, there are more than a few delegates on the line here. A strong showing in Iowa will serve as validation for their theory of how to run a successful campaign at a time when Democrats, and the left more broadly, is searching for ways to forge a new progressive coalition.
Other camps, typically the moderates, argue that more affluent suburban voters hold the keys to Democrats' reclaiming the presidency, much in the same way they helped deliver a House majority last year.
Sanders isn't writing off those groups or those parts of the state, but if he is going to win on Monday, it will be on the strength of a big, broad and diverse electorate turning up, and turning out for him.
What if progressives get what they've been wishing for?
Progressive leaders with an affinity for both Sanders and Warren would like nothing more than to see them run 1-2 on Monday night, a result that would by definition mean Biden, Buttigieg and Klobuchar underperformed, and signal the strength of the left going forward.
Though they tend to pull support from different pools of voters, Sanders and Warren remain the progressive movement's most recognizable -- and, at least among those most inclined to their politics -- beloved figures. To see them dominate the caucuses would be a shot of adrenaline that could carry into New Hampshire, a state that borders both of their own.
But in the end, of course, there can only be one. And if Warren's and Sanders' campaigns leave Iowa with impression that their next step on the road to a potential nomination is to consolidate progressives, the movement might have some cause for concern.
Warren and Sanders are friends, as they both have been increasingly quick to remind reporters in the wake of the scuffle over the contents of their now-famous 2018 meeting, but they are also both running massive campaign operations with differing messages. Warren has in recent weeks played up her ability to unite Democratic factions. Sanders, though repeatedly promising to back the eventual nominee, does not make that distinct argument. His goal, in stark terms, is to defeat the establishment and then count on its support when he faces off with Trump.
So the progressive nightmare scenario is closely attached to the dream one. It looks like a divisive campaign pitting the left's standard-bearers against one another and, in the process, clearing a path for a moderate resurgence.