DES MOINES, Iowa — Iowans were set Monday to let the rest of the world know their preferred choice for the 2020 Democratic presidential nomination.

The candidates in the hotly contested race spent the day making last-second pitches and rallying their campaign troops.

Iowa’s first-in-the-nation presidential precinct caucuses took place Monday night across the state. The Democratic race had been close for months, and the top four candidates in the polling — Sen. Bernie Sanders, I-Vt.; former Vice President Joe Biden; former South Bend, Indiana, Mayor Pete Buttigieg; and Sen. Elizabeth Warren, D-Mass. — were expected to have a chance to win the opening state in the nation’s presidential nominating process.

That quartet has been bunched atop polling on the race for months, and each has built a campaign organization strong enough to be successful here.

Iowans took their decision down to the wire: Polling even in the final week of the caucus campaign showed roughly half of Iowa Democrats remained either undecided or willing to have their minds changed about their preferred candidate.

Most of the precinct caucuses were scheduled to begin at 7 p.m. Central. Some satellite caucuses were already taking place Monday afternoon.

On the day of the caucuses, Sanders held a slim lead in Real Clear Politics’ rolling average of polling on the race in Iowa; with all of the polls reported, Sanders was at an average of 23%, Biden at 19.3%, Buttigieg at 16.8% and Warren at 15.5%.

A poll published Monday morning by the liberal advocacy group Focus on Rural America showed Buttigieg at 19%, Sanders at 17%, Biden and Warren at 15% each and Sen. Amy Klobuchar, D-Minn., at 11%.

“The margin of error places the top five contenders … in a virtual tie,” the group said.

In other words, even in the final hours of a campaign that has lasted for more than a year, Iowa remained up for grabs.

This year’s caucuses also brought new ways of measuring candidates’ success. For the first time, the state party was set to announce not just the state delegate equivalents awarded to each candidate at the end of each precinct caucus, but also the first preference counts for each candidate.

In the Democratic caucuses, supporters gather in groups for each candidate, then some reorganizing often takes place based on the candidates’ varied levels of support.

Previously, the state party did not report how much support each candidate received in that first preference round. This year, the party will release that number. That could impact the way this year’s results are interpreted by the campaigns and the media.

The state party will also announce a third figure: the final realignment. But with some minor exceptions, that figure should closely match the state delegate equivalents.

“We could have three winners,” David Redlawsk, a political science professor at the University of Delaware, a visiting professor at the University of Iowa and author of a book on the Iowa caucuses, said during a recent episode of “Iowa Press” on PBS.

Because the party’s nominee is eventually chosen based on delegates won, the state party urged media outlets and caucus-watchers to use the state delegate equivalent counts to determine the caucus winner.

Most experts and veterans of the Iowa caucus expected a record turnout. The previous high was 236,000, set in the 2008 Democratic caucuses that started Barack Obama’s trajectory to the White House.

Interest and enthusiasm have been high at candidate events throughout the past year, but especially during the final weeks before the caucuses. More than 3,000 attended a Sanders event over the weekend in Cedar Rapids, and Buttigieg, Warren and Biden all have drawn four-figure crowds.

The candidates tried to use Monday’s final moments before the caucuses to rally last-minute support.

Buttigieg made an appearance at his campaign’s field office in West Des Moines to energize campaign staff and volunteers.

“It feels great out there, and I don’t even know what time it is,” Buttigieg joked Monday morning. “You are part of an absolute force that is sweeping through the state of Iowa right now.”

A Buttigieg staff member told volunteers, before they headed out for one last day of door-knocking, to not only encourage Iowans to support Buttigieg but to encourage anyone who may prefer a different candidate to consider Buttigieg as their second choice.

Iowans’ second choices could play a significant role in the outcome.

The supporters of any candidate that fails to become viable in the caucus — receive at least 15% support, in most precincts — are free to move to another candidate. So the second choices of candidates who fail to become viable could help sway the results in such a tight race.

Warren, who along with her fellow U.S. senators was stuck in Washington for the impeachment trial of Republican President Donald Trump, held a telephonic town hall with Iowa supporters Monday morning.

Warren fielded a few questions and made her pitch as the Democratic candidate who can defeat Trump in November’s general election. She said she has won an election against a Republican incumbent, noted the gains made by Democrats in the 2018 midterms in part due to victorious female candidates and said her anti-corruption plans that are at the heart of her candidacy present a platform on which Democrats can run and win.

“Every one of those plans I’ve described is popular not just with Democrats, but with Republicans and independents,” Warren said. “That’s how we pull our party together and that’s how we pull in some Republicans and independents, and that’s how we will beat Donald Trump.”