(AP) -- In a high-stakes test of enthusiasm versus organization, Republican Donald Trump and Democrat Bernie Sanders hope to ride voter energy into victories in Monday's Iowa caucuses, as Ted Cruz and Hillary Clinton bank on sophisticated get-out-the vote operations months in the making.

The caucuses kick off the 2016 presidential nominating contests, marking a new phase in a tumultuous election that has exposed Americans' deep frustration with Washington and given rise to candidates few expected to challenge for their party's nomination when they first entered the race.

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, Trump appears to hold a slim edge over Cruz, a fiery senator from Texas. Clinton and 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.

"Stick with me," Clinton said as she rallied supporters Sunday in Council Bluffs. "Stick with a plan. Stick with experience."

Sanders, the Vermont senator who has been generating big, youthful crowds across the state, urged voters to help him "make history" with a win in Iowa.

In a show of financial strength, Sanders' campaign announced Sunday it had raised $20 million in January alone. While Sanders has a large team in Iowa, his operation got off to a later start, particularly compared with Clinton, who has had staff on the ground in the state for nearly a year.

Monday's contest will also offer the first hard evidence of whether Trump can turn the legion of fans drawn to his plainspoken populism into voters. The scope of the billionaire's organization in Iowa is a mystery, though Trump himself has intensified his campaign schedule during the final sprint, including a pair of rallies Monday.

Cruz has modeled his campaign after past Iowa winners, visiting all of the state's 99 counties and courting influential evangelical and conservative leaders. With the state seemingly tailor-made for his brand of uncompromising conservatism, a loss to Trump will likely be viewed as a failure to meet expectations.

Seeking to tamp down expectations, Cruz said Sunday that he's just pleased to be in the mix for first place.

"If you had told me a year ago that two days out from the Iowa caucuses we would be neck and neck, effectively tied for first place in the state of Iowa, I would have been thrilled," Cruz said.

Cruz has spent the closing days of the Iowa campaign focused intensely on Marco Rubio, trying to ensure the Florida senator doesn't inch into second place. Rubio is viewed by many Republicans as a more mainstream alternative to Trump and Cruz, though he'll need to stay competitive in Iowa in order to maintain his viability.

The campaigns were anxiously keeping an eye on the weather. A snowfall forecast to start Monday night appeared more likely to hinder the hopefuls in their rush out of Iowa than the voters. Republican John Kasich already had decamped to New Hampshire, with New Jersey Gov. Chris Christie and former Florida Gov. Jeb Bush following behind Monday afternoon, hours before the caucuses start.

The trio of governors has had a light footprint in Iowa, banking instead on strong showings in New Hampshire's Feb. 9 primary to jumpstart their White House bids. Yet some Republican leaders worry that if Trump or Cruz pull off a big victory in Iowa, it would be difficult to slow their momentum.

Bush, for example, started the year as a fundraising juggernaut. But according to records released Sunday, both his super PAC and campaign fundraising declined significantly in the later months of 2015 as he struggled to keep up with Trump.

Unlike in primaries, where voters can cast their ballots throughout the day, the caucuses begin across Iowa at 7 p.m. CST. Democrats will gather at 1,100 locations and Republicans at nearly 900 spots.

Turnout was expected to be high. The Iowa Republican Party expected GOP turnout to top the previous record of 120,000 people in 2012. Democrats also expect a strong turnout, though not nearly as large as the record-setting 240,000 people who caucused in the 2008 contest between Clinton, Barack Obama and John Edwards.

Iowa has decidedly mixed results in picking the parties' eventual nominees. The past two Republican caucus winners -- former Arkansas Gov. Mike Huckabee and former Pennsylvania Sen. Rick Santorum -- faded as the race stretched on. But Obama's unexpected 2008 victory was instrumental in his path to the nomination, easing the anxieties of those who worried the young black senator would struggle to win white voters.

While both parties caucus on the same night, they do so with different rules.

Republicans vote by private ballot. The state's 30 Republican delegates are awarded proportionally based on the stateside vote.

Democrats take a more interactive approach, with voters forming groups and publicly declaring their support for a candidate. If the number of people in any group is fewer than 15 percent of the total, they can either choose not to participate or can join another viable candidate's group.

Those numbers are awarded proportionately, based on statewide and congressional district voting, as Iowa Democrats determine their 44 delegates to the national convention.

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Pace reported from Washington. Associated Press writers Lisa Lerer, Scott McFetridge and Scott Bauer contributed to this report.

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