DES MOINES, Iowa — The election year 2020 began the same way 2019 did: with former Vice President Joe Biden and Sen. Bernie Sanders, I-Vt., leading in national polls.

But, if anything, the narrow lead Biden and Sanders hold over the competitive Democratic primary field has left the race looking more muddled than ever and many voters remain undecided ahead of the first-in-the-nation Iowa caucuses Feb. 3.

A lot could change in the next three weeks, though, even if little did over the past 52.

Iowa Democrats, many of whom have been preoccupied with President Donald Trump's impeachment, will be forced soon to stop shopping for a candidate and settle on the person they want to go up against the president in November.

"A world created in a week can change in three," Philippe Reines, a former top aide to Hillary Clinton, said.

A new Des Moines Register poll out Friday showed the four leading candidates — Biden, Sanders, Sen. Elizabeth Warren, D-Mass., and former South Bend, Indiana, Mayor Pete Buttigieg — within the margin of error of each other. And some 6 in 10 likely caucusgoers say they're still open to being persuaded.

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"Everybody keeps asking who is going to win," Troy Price, the chairman of the Iowa Democratic Party, said. "I tell them: 'I have no earthly idea.'"

Iowa political veterans say this year's nominating contest is especially turbulent, with a number of key variables almost impossible to predict. Here are five things they're watching:

1. Turnout

Iowa Democrats are preparing for record-breaking turnout, with more and larger venues for the 1,679 precincts where caucus meetings will take place across the state (and around the world this year with the addition of satellite caucuses for expats as far away as Tbilisi, the capital of Georgia).

But turnout estimates are all over the map and voter registration numbers show a decline among Democrats as the state trends Republican (there are 614,519 registered active Democrats now, compared to 631,136 this time last year, according to the Iowa secretary of state).

How many voters participate in this year's caucuses could be crucial.

Barack Obama's surprise Iowa victory in 2008 was powered by an unexpected turnout surge, which overwhelmed the carefully projected models of Hillary Clinton's campaign.

It was a similar story for Sanders' near-win in Iowa in 2016, which many observers didn't see coming because the Vermont independent inspired people to participate who don't normally show up to caucus.

"Back in 2016 (Bernie) really surprised me in terms of the number of people he brought out, and I think a number of people are still with him," said Bret Nilles, chairman of the local Democratic Party in Linn County, which includes Cedar Rapids.

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Meanwhile, with no contest on the GOP side, some Republicans and independents say they plan to attend the Democratic caucuses next month (same-day registration is allowed), which could make a difference in small rural precincts where even a few extra people showing up can swing things.

2. Organization

The campaigns' "ground games" — getting their supporters out to caucus and having skilled staffers at the precincts — only matters on the margins, but that could be the whole game in a tight race.

Caucuses in Iowa are nothing like the quiet privacy of a voting booth. They are rowdy, complicated and personal, since everyone has to physically gather together. Lobbying for candidates inside the room is not only allowed, but also expected, so it's critical for campaigns to have representatives in each precinct.

And the rules are more complicated than normal voting, so campaigns with trained precinct staffers who know how to exploit them can gain an advantage over more disorganized campaigns.

"What might 'change' is what's invisible until caucus night — what kind of organization the campaigns are building," former Clinton 2016 campaign manager Robby Mook said. "It takes months to build. You can't slap it together overnight."

Some Iowans say this could be Warren's saving grace, since she is widely seen as having the most robust field organization. Other campaigns are still scrambling to recruit representatives for each caucus site.

"It is that one-on-one voter contact that can push a voter over the edge for a certain candidate," Olivia Habinck, the president of College and Young Democrats of Iowa, said. "Finding precinct captains that are able to convince the undecided voters on caucus night to align with their candidate will be crucial."

3. Fence-sitters, late-breakers

With so many candidates in the race and Trump dominating the news, many Iowa Democrats have been waiting to see how things would shake out before getting serious about picking a candidate.

"People are still trying to make up their mind," Nilles said. "They have their top two or three, but you're going to see a significant amount of people making up their mind here in the last few weeks."

Some voters have said they could make their decision in the caucus room as their neighbors try to sway them and once they see which of their favorites has the most support. Building a perception of momentum heading into caucus night could be helpful in wooing the late-deciders, strategists say.

"Everybody's jumbled in there all together and the last week is so really critical," former Secretary of State and Democratic presidential nominee John Kerry told NBC News while stumping for Biden in Cedar Rapids last week.

Kerry himself was a surprise winner of the caucuses in 2004, surging to victory as Democrats rallied behind someone they viewed as the safest choice to take on then-Republican President George W. Bush.

4. "Realignment" and dealmaking

The most important number in the caucuses is 15 percent. That's the threshold of support candidates need to reach to be deemed "viable" in each precinct; otherwise their supporters will be forced to disband and "realign" with another candidate or join the undeclared group.

According to the Des Moines Register poll, only the top four candidates can feel confident about meeting the viability threshold (and Biden was right on the bubble at 15 percent).

That doesn't mean someone like Minnesota Sen. Amy Klobuchar (at 6 percent in the poll) will be shut out. It goes caucus by caucus and there will likely be precincts where Klobuchar is above the threshold, such as in northern towns closer to the border with her home state. The same is true for other candidates, as well.

But it does mean that a lot of voters will be up for grabs in the room on caucus night if and when their preferred candidates don't make the threshold.

"Each candidate has their base and their base is not going to move, so they’re going to have to pick from others," who don't reach viability, said Ann Anhalt, a Des Moines Republican who plans to participate in a Democratic caucus. "That's kind of what is unknown now and what could change, but that will all come down to caucus night."

It's not uncommon for campaigns to cut secret deals with each other to send their supporters one way or another if their candidate comes up short in a given caucus.

5. Impeachment

With Trump's impeachment trial soon to kick off in the Senate, the five Democratic senators running for president will likely wish they had a clone, since they'll be in Washington to serve as "jurors" while also needing to campaign in Iowa.

It's a scheduling nightmare for some candidates, like Sanders, Warren and Klobuchar, and a dream for others, like Biden and Buttigieg, who may get the state virtually to themselves.

Campaigns are exploring all kinds of contingency options, such as sending high-profile supporters to lead events or chartering private jets to zip back and forth between Iowa and Washington, but nothing can replace the face time.

"It makes it an uneven playing field, but I am hopeful that surrogates will come out," Edwin Purcell, a committed Sanders supporter from Newton, said.

Mook, Clinton's former campaign manager, said anything can still happen Feb. 3.

"Most voters are fundamentally ambivalent," he said. "They just want the best nominee to beat Trump ... It's unclear who that is."

Alex Seitz-Wald reported from Washington, and Lauren Egan reported from Iowa.