DES MOINES — Donald TrumpDonald TrumpKushner: Meeting with congressional investigators went 'very well' The Hill's 12:30 Report Newsweek settles with Sputnik writer MORE is about to meet his moment of truth.

The Iowa caucuses on Monday evening offer the first opportunity for voters to weigh in on a presidential race that Trump has transformed since declaring his candidacy more than seven months ago.

If his supporters come out in large enough numbers to deliver him victory, it will prove beyond doubt that the Trump phenomenon is real. It will also mean that his march toward the nomination will be hard to stop.

Just the opposite could happen, too. A loss for Trump would suggest that his brio and personal celebrity is not backed by enough organizational muscle to get results.

Which will it be? Even experts steeped in Iowa Republican politics admit they simply don’t know.

Matt Strawn, who served as chairman of the Republican Party of Iowa from 2009 to 2012, says there is a “giant, Trump-sized question mark” about whether the people who profess support for the business mogul will show up.

“There are these large, sweeping crowds, but what kind of mechanism has the campaign created to turn those event-attenders into caucusgoers?” asked Strawn, who is not affiliated with any candidate’s campaign.

An aide to a rival campaign who spoke on condition of anonymity said the issue of turnout for Trump was “the 20 million dollar question” with which everyone was grappling.

Trump led Ted CruzTed CruzGOP wrestles with soaring deductibles in healthcare bill Cruz: Tax reform chances ‘drop significantly’ if healthcare fails Ex-CBO directors defend against GOP attacks on ObamaCare analysis MORE, his main rival here, by 5 points in the final Des Moines Register/Bloomberg poll, which was released on Saturday evening. Cruz had held a 3-point lead in the previous version of that poll, which was released in mid-January. Trump is also ahead in the RealClearPolitics (RCP) polling average in the state, with an edge of 6.2 percentage points.

At a weekend rally in a theater in the eastern Iowa city of Davenport, Trump told the audience, "We really are on the cusp of something so big.” But, he added, "it all doesn't matter if you don't caucus on Monday. The polls don't matter, nothing matters.”

In an interview broadcast on CBS’s “Face The Nation” on Sunday, Trump said that he didn’t “have to” win Iowa but added, “I think we have a good chance of winning Iowa. I’d like to.” Former Republican vice presidential candidate Sarah Palin will join him in the state on Monday as part of that effort.

Several Iowa experts have told The Hill that Trump’s rivals underestimate his ground-game at their peril. 

But others, such as Strawn, note that there is “very little empirical evidence” of the kind of surge in voter registration that might be expected if a tsunami of first-time caucusgoers was going to materialize.

That doesn’t mean Trump is doomed to failure — for one thing, it is possible to register on-site the night of the caucuses. But it does deepen the questions over his chances.

He is not the only candidate to be depending on new voters, however.

Sanders seeks a surge

Bernie SandersBernie SandersGOP Senate candidate uses Scalise shooting in ad The Hill's 12:30 Report Report: minimum wage bill would benefit 20.7 million workers in 21 states MORE, a self-described democratic socialist, is diametrically different from Trump in almost every respect. But he needs a surge in first-time caucusgoers among Democrats just as the mogul requires one among Republicans.

Sanders lags Democratic front-runner Hillary ClintonHillary Rodham ClintonKushner: Meeting with congressional investigators went 'very well' The Hill's 12:30 Report Scarborough: Trump sounds 'beleaguered and frightened' MORE by 3 points in the Des Moines Register poll and by an almost identical margin in the RCP average. His aides argue that his message — in essence, that the nation needs a more radical shift to the left than Clinton would provide — will inspire liberal Iowans, drive turnout beyond the pollsters’ expectations and notch a win.

“If there is a large voter turnout — if those people who previously did not vote … come out, we win. If there is a low voter turnout, we lose. That’s a fact,” Sanders told a Davenport rally on Friday.

The event was attended by around 1,000 people, many of whom were clearly true believers in the Sanders cause. As the candidate spoke about people having to cut medical pills in half because they couldn’t afford to do otherwise, a bearded man in late middle-age turned to the assembled media, saying emphatically, “That’s me, that’s me.” 

That same night, at an event just blocks away addressed by Clinton and her husband, former President Bill Clinton, attracted a crowd that was less raucous — but significantly larger — than the one that came to hear Sanders.

Hillary Clinton has been locking President Obama in a tight metaphorical embrace, emphasizing her support for him at every turn. “Stick with me!” is her refrain at rallies in the closing days.

Barbara Trish, a political science professor at Grinnell College in Iowa, said that she believed “there was something about the Sanders candidacy — the outsider railing against the establishment — that appeals not just to young people but to liberals in general.” But she noted that Clinton had her enthusiasts too.

Cruz puts faith in ground-game

Cruz and his aides believe they have a clear shot at victory here, despite a softening of the Texan’s poll numbers as he has come under sustained attack from Trump, other candidates and their allies.

That faith rests on the belief that they have the best organized effort of any Republican candidate — an opinion that is widely shared by independent experts.

In particular, Cruz’s strength among evangelicals could be pivotal, since those people are among the most reliable caucusgoers on the Republican side. At a rally in Des Moines Sunday night, Cruz's speech contained several biblical references as he urged his supporters to "awaken the body of Christ to pull us back from the abyss." 

(Not all his references were so sacred. When a heckler toward the rear of the crowd briefly interrupted the start of his speech, Cruz responded, "Is that Donald Trump down there in the back?") 

“Ted Cruz’s support is largely built on people who caucus every four years like it’s their job,” said Strawn, the former Republican Part of Iowa chairman. “Whether it’s through 3-feet snowdrifts or not, they’ll be there.”

Rick Tyler, the Cruz campaign’s communications director, told The Hill that he was “optimistic” of victory because of the campaign’s on-the-ground organizing.

“That’s how I think Iowa is traditionally won, and we don’t think that has changed,” he said.

Rubio and the rest

Marco RubioMarco RubioMexican politicians have a new piñata: Donald Trump Bush ethics lawyer: Congress must tell Trump not to fire Mueller The private alternative to the National Flood Insurance Program  MORE, the third-place Republican here and nationally, has been in Iowa throughout the past week, and his aides talk up the idea that he has momentum leading into Monday evening. 

Some independent experts acknowledge that Rubio is well-liked even among GOP supporters for whom he might not be a first choice. And the Cruz campaign has attacked him with enough vigor to suggest they are worried about his strength. 

In other quarters, though, there is considerable skepticism about the idea of a Rubio surge, which has been predicted several times without ever clearly coming to pass. He advanced only modestly in the latest Des Moines Register poll.

For other candidates, survival is the name of the game. Establishment-friendly Republicans — Chris Christie, John Kasich and Jeb Bush — are largely competing against each other, seeking to shape a favorable perception as the more hospitable territory of New Hampshire beckons. The Granite State votes on Feb. 9. 

Meanwhile, the end of the road is likely near for Rick Santorum and Mike Huckabee, social conservatives who need a strong Iowa result and almost certainly won’t get one. 

Rand Paul, Ben Carson and Carly Fiorina are mainly hoping just to outperform low expectations.

On the Democratic side, Martin O’Malley is trailing way behind Clinton and Sanders. But he could end up being a factor if he ultimately directs his supporters to one of the big two.