DES MOINES, Iowa (AP) — A Democratic campaign that has cost more than $1 billion, dashed the ambitions of veteran politicians, forced conversations about race, gender and identity and prompted fierce debate over health care and taxes crests Monday in the Iowa caucuses.

By day's end, tens of thousands of Democrats will have participated in the famed Iowa caucuses, the premiere of more than 50 contests that will unfold over the next five months. The caucuses will render the first verdict on who among dozens of candidates is best positioned to take on President Donald Trump, whom Democratic voters are desperate to beat this fall.

It is a moment thick with promise for a Democratic Party that has seized major gains since Trump won the White House in 2016. But instead of optimism, a cloud of uncertainty and deepening intraparty resentment hangs over Monday’s election, which, after a multi-year buildup, will finally begin to reveal who and what Democrats stand for in this tumultuous era.

MORE: Additional coverage of the Iowa Caucuses on Cheddar.com

“If anybody tells you they know who’s going to win, either they’ve got a whisper from God or they’re loony, because nobody knows,” said Deidre DeJear, the former state chair for Kamala Harris and the first black woman to win a statewide primary in Iowa.

Polls suggest that Vermont Sen. Bernie Sanders may have a narrow lead, but any of the top four candidates — Sanders, former Vice President Joe Biden, Massachusetts Sen. Elizabeth Warren and former South Bend, Indiana, Mayor Pete Buttigieg — could score victory in Iowa’s unpredictable and quirky caucus system as organizers prepare for record turnout. Sen. Amy Klobuchar, who represents neighboring Minnesota, is also claiming momentum, while outsider candidates such as entrepreneur Andrew Yang, billionaire activist Tom Steyer and Hawaii Rep. Tulsi Gabbard could be factors.

Iowa offers just a tiny percentage of the delegates needed to win the nomination but plays an outsize role in culling primary fields. A poor showing in Iowa could cause a front-runner’s fundraising to slow and support in later states to dwindle, while a strong result can give a candidate much needed momentum that propels him or her to the nomination.

The past several Democrats who won the Iowa caucuses went on to clinch the party’s nomination.

The 2020 fight has played out over myriad distractions, particularly congressional Democrats’ push to impeach Trump, which has often overshadowed the primary and effectively pinned several leading candidates to Washington at the pinnacle of the early campaign season. Even on caucus day, Sanders, Warren and Klobuchar were expected to spend several hours on Capitol Hill for impeachment-related business.

Meanwhile, New York ultra-billionaire former Mayor Mike Bloomberg is running a parallel campaign that ignores Iowa as he prepares to pounce on any perceived weaknesses in the field come March.

The amalgam of oddities, including new rules for reporting the already complicated caucus results, is building toward what could be a murky Iowa finale before the race pivots quickly to New Hampshire, which votes just eight days later.

With uncertainty comes opportunity for campaigns desperate for momentum. The expectations game was raging in the hours before voters began gathering at high school gyms and community centers in more than 1,600 caucus sites across the state.

Having predicted victory multiple times in recent weeks, Biden’s team sought to downplay the importance of Iowa’s kick-off contest the day before voting began amid persistent signs that the 77-year-old lifelong politician was struggling to raise money or generate excitement on the ground.

Biden senior adviser Symone Sanders said the campaign viewed Iowa “as the beginning, not the end,” of the primary process.

“It would be a gross mistake on the part of reporters, voters or anyone else to view whatever happens on Monday — we think it’s going to be close — but view whatever happens as the end and not give credence and space for New Hampshire, Nevada and South Carolina,” she said of the three states up next on the primary calendar.

The tone was noticeably more upbeat for Bernie Sanders’ campaign, which has repeatedly predicted victory and believes he’s running even stronger in New Hampshire. That’s despite increasingly vocal concerns from establishment-minded Democrats who fear the self-described democratic socialist would struggle against Trump and make it more difficult for Democrats to win other elections this fall.

In a fundraising message, Sanders campaign manager Faiz Shakir warned supporters to expect “an absolutely huge barrage of attacks from the political establishment” after Monday’s results are announced, implying again that Sanders would finish on top.

“We don’t know how personal the attacks will be. We don’t know how many millions will be spent. And we certainly won’t know who’s funding them until well after the damage is done,” he said. “But we know they’ll come. Because they want to stop Bernie. They want to stop our movement.”

The heated rhetoric underscores a dangerous rift between Sanders’ passionate supporters and other factions of Democrats who have clashed in recent days but must find a way unite should they hope to defeat Trump in November.

Joe Trippi, campaign manager for 2004 presidential candidate Howard Dean, suggested that Sanders may have peaked too soon, forcing some voters to seek a less controversial standard bearer.

“There are a lot of reasons this thing moves hard at the end,” Trippi predicted. “I think Bernie going into the lead is really kind of the disruptive thing that’s going to make a lot of people look at who they are for.”

New party rules may give Sanders and his rivals an opportunity to claim victory, even if they aren’t the official winner.

For the first time, the Iowa Democratic Party will report three sets of results at the end of the night: tallies of the “first alignment” of caucus-goers, their “final alignment” and the total number of state delegate equivalents each candidate receives. There is no guarantee that all three will show the same winner.

The Associated Press will declare a winner based on the number of state delegates each candidate wins, which has been the traditional standard.

While Sanders and Biden are central figures in the race, Warren and Buttigieg are fighting to prove they can assemble the coalition required to win the nomination as well.

Warren, who has leaned into her status as one of the only female candidates in recent weeks, was offering free child care for voters at some caucus locations as her campaign used every available tool — even online dating apps — to court supporters.

Buttigieg, a 38-year-old former city official who had virtually no national profile a year ago, walked into Monday as a legitimate threat to win the nomination. In fact, he predicted victory as he faced a frenetic audience of more than 2,000 people at a high school gymnasium the day before the caucus.

Early Monday, reacting to a Trump campaign Super Bowl ad about criminal justice reform, Buttigieg said it’s one of the handful of things Trump has done that he agrees with.

“It doesn’t change the incredible cruel and divisive racial rhetoric that comes out of this White House, which is one of the many reasons that I’m meeting not only Democrats but Republicans who tell me they struggle to look their children in the eye and explain to them how this is the president of the United States,” Buttigieg said on Fox News Channel.

Though most of the attention will be on Democrats, Republicans will also hold caucuses on Monday. With no serious challenger and plenty of money to burn, Trump’s reelection team hopes to use voting in early states as a test run for its organizing prowess and to boost excitement for the president’s fall campaign. Trump held a rally in Iowa last week and dispatched surrogates to the state ahead of Monday’s vote.