Freight train pulls flatcars carrying combine harvesters through downtown Ogden, Iowa (Luke Sharrett/Bloomberg)
Chief correspondent

The final days before the Iowa caucuses have produced a grand spectacle. Venues are filled to capacity and beyond. Cars clog parking lots, with many others stacked along nearby roadways. Camera crews fight for space on the risers, and the huge crush of journalists look for new scraps to freshen their stories.

In its splendor, these last days and hours represent a display of democracy in action — under the brightest lights imaginable. And yet, on a day when, at long last, the first votes of the 2016 presidential campaign will be cast, the same enduring questions surround the results: How much will Iowa matter? And should it?

The reality is that, no matter the criticism of the process, Iowa always matters. As the first event, it is hardly decisive, but its role in winnowing the field and offering clues as to the candidates’ core supporters is important. More than anything, it marks the line of demarcation when the voters begin to have their say.

Over four decades, the caucus process has been transformed from small gatherings of party activists into something more akin to a primary campaign, but which culminates in an unusual, and to some people an arcane and undemocratic, exercise for a few hours on a Monday evening.

The Republican caucuses are straightforward. Voters assemble at a specified time, possibly listen to speeches on behalf of many candidates in the race, cast their votes by secret ballots and are free to depart. The ballots are tabulated and reported; the percentages are of the total number of voters who participated.

With candidates vying neck-and-neck for support in Iowa, the Post’s David Weigel walks through what it will take to win the Iowa Caucuses. (Alice Li/The Washington Post)

It’s not so simple for the Democrats. Voters make their declarations publicly. No secret ballots here. Voters are asked to stand and deliver. The process is an exercise of conviction, persuasion and horse-trading. Results are produced by a complex formula and reported to the public not as percentages of the raw vote each candidate has received, but by what the Iowa Democratic Party calls “state delegate equivalents,” or simply “SDEs.”

Please do not ask for a more detailed description. Just remember that the percentages for Hillary Clinton and Sen. Bernie Sanders will be a close approximation of the actual support each had on caucus night, not a precise percentage of the vote. Former Maryland governor Martin O’Malley, who hovers in single digits in the late polls here, might be wiped out entirely if he fails to reach the necessary threshold to qualify for delegates in the 1,681 precincts.

There’s no question that both the Democratic and Republican caucuses deny some people the opportunity to participate. Unlike a primary, when polls are open from dawn to dark, there is but one window for taking part in the caucuses. There are new provisions this year to make it easier to participate for some who otherwise might be unable to do so. But it is by nature limiting and, to those not closely aligned with their party, it can be intimidating and seemingly exclusionary.

Not just ordinary citizens have a resistance to the caucuses. Hillary and Bill Clinton left Iowa in 2008 with a sour taste about the process. No doubt that had much to do with her third-place finish that year behind then-Sen. Barack Obama (Ill.) and former senator John Edwards (N.C.). But Bill Clinton in particular never seemed to get over his dislike for the process here, feeling that it was anti-democratic and given to chicanery.

Beyond the caucus process, Iowa’s place at the front of the line draws continuing scrutiny and criticism. Why should a state that is not demographically representative of the country have such a coveted place in the primary-caucus calendar? The U.S. Census Bureau shows that 92 percent of Iowans are white, compared with 77 percent nationally.

For Democrats in particular, the lack of diversity is an issue that the party has wrestled with for years, resulting in efforts to boost the role of South Carolina and Nevada after the Iowa and New Hampshire contests. Republicans also have an issue because the caucus electorate, with its preponderance of religious conservatives, is more conservative than the party as a whole.

Christian conservatives made up 57 percent of the GOP caucus electorate in 2012, which has caused most mainstream Republican candidates to avoid or play down their investment in the state, hoping to discount the results. The final Des Moines Register-Bloomberg Politics poll showed all those mainstream conservatives, with the exception of Sen. Marco Rubio (Fla.), in low single digits, a shockingly poor performance.

Journalists cover a Donald Trump rally on Sunday in Council Bluffs, Iowa. (Charles Ommanney/The Washington Post)

What has kept Iowa in the forefront is the role that citizens here have played in vetting the candidates over many campaigns. There are no other states beyond Iowa and New Hampshire where people take as much time to seek out the candidates, listen to them, question them and evaluate them.

The people here can be demanding, both in the attention they expect to be paid and in the probing nature of their questions. But in the last cycle and in particular this one, some of the intimacy of the past has begun to disappear. Donald Trump’s huge rallies — he rarely takes questions from the audience — may be singular to his candidacy, but they are a break with custom and tradition and could erode what has been especially valuable about campaigning in Iowa.

Still, for all the questions about the process, Iowa matters. It matters in part because it is the first test, coming after almost a full year of campaigning, polling, punditry and the like. Voters almost always change things, in small or large ways. The campaigning here already has shaped the races. Monday’s results will further shape New Hampshire, and the Granite State will shape South Carolina and beyond.

The results matter both for the actual order of finish and the way those results stratify the field. On the Republican side, the past two caucuses have produced winners — Mike Huckabee in 2008 and Rick Santorum in 2012 — who were extremely popular with evangelical voters but who couldn’t go the distance. For that reason alone, Iowa’s place at the front of the line is in question.

The quote long attributed to the football legend Vince Lombardi — “Winning isn’t everything, it’s the only thing” — doesn’t hold in Iowa. Finishing second or third is often almost as good as finishing first. Since the caucuses took hold among Republicans in 1980, just two caucus winners have gone on to become the GOP nominee: Bob Dole in 1996 and George W. Bush in 2000.

Losing badly here can be a problem, unless it isn’t. Sen. John McCain finished fifth here in 2000 with 5 percent of the vote, but he redeemed himself the following week by defeating Bush by 19 points in New Hampshire and turning the GOP race into a pitched battle. He finished fourth here in 2008 and went on to win the nomination.

In both cases, however, McCain spent virtually no time in Iowa. That’s not been the case this year for former Florida governor Jeb Bush and New Jersey Gov. Chris Christie. They have invested far more in New Hampshire, but Bush in particular has spent considerable time in Iowa and his super PAC has outspent all others here — $15 million in TV ads, according to Mark Murray of NBC News. He will have no good excuses for a poor finish.

Sometimes the winner isn’t the ultimate story coming out of Iowa. In 1984, Democrat Walter F. Mondale won the caucuses with 49 percent. Gary Hart finished a distant second at 17 percent and became the story. Two weeks later, after Hart won the New Hampshire primary and the Maine caucuses, Mondale’s candidacy was in deep trouble. Mondale went on to win, but it took him the rest of the primary season to do so.

What has already happened here is important. The large Republican field is being winnowed quickly. Right now it looks as if Trump, Sen. Ted Cruz (Tex.) and Rubio will claim the traditional three tickets out of the state. Other Republicans will need a quick turnaround in New Hampshire to get into the race. Meanwhile, Iowa Democrats have turned Sanders from a protest vehicle for progressives into a real challenger to Clinton.

Iowa’s process isn’t perfect by any means. The results won’t necessarily predict the outcome of the nomination contests. But for anyone who wonders, it still matters.