DES MOINES, Iowa – Former New York City Mayor Michael Bloomberg zigzagged across the state in one day this week to talk about renewable energy and climate change. He premiered his new film, “Paris to Pittsburgh” to a local audience here.

A few months earlier, New Jersey Sen. Cory Booker came here to tout his roots and fire up the crowds. California Sen. Kamala Harris spoke there and paid a visit to Bryce Smith, the Democratic chairman of one of Iowa’s most populous counties. Vermont Sen. Bernie Sanders and former Vice President Joe Biden also visited Iowa recently.

Democrats mulling a run for the presidency in 2020 are already flocking to the Hawkeye State to test the waters before officially announcing their candidacy. Iowa is the center of attention right now for a long list of presidential contenders – and Democratic leaders in the state are playing a critical role in trying to figure out who should make the cut.

“If you finish in the top three coming out of Iowa, there’s a high likelihood you’ll be the nominee of the party…We don’t pick the one who is the eventual party nominee, but we do help to whittle the field,” said former Polk County Democratic Party Chairman Tom Henderson.

Since the midterms, potential Democratic contenders have been making moves in the state—and for good reason. Competition is expected to stack up to a massive field of at least two dozen candidates, some of whom have been focused on Iowa since just after the 2016 election.

“People don’t come to Iowa for the weather, certainly potential political candidates don’t,” joked political analyst Dennis Goldford. “Iowa is not first because it’s important. Iowa is important because it’s first. The caucus is a testing period—it’s as if you built a race car and in Iowa, you put it on the track to see how it runs.”

Bloomberg, who has not officially declared his candidacy, has been aggressive in trying to woo Iowa Democrats. He penned a guest column in The Des Moines Register, the state’s largest newspaper, before his trip to Iowa, referencing the 2020 race.

The recently re-registered Democrat has toyed with a bid for the White House for the past two election cycles, but after his $250,000 donation to the Iowa Democratic Party this year and his PAC’s $100 million investment into helping Democrats regain control of the House in the midterm elections, speculation has grown over the 76-year-old businessman’s intentions. Iowa traditionally hosts the nation’s first contest of the presidential primary season, set for February 2020.

“I’ve said I will look at it after the first of the year,” he told Fox News on Tuesday, “and we’ll see then.”

A year from now, the state will be the country’s political epicenter. But Iowa didn’t always play such an important role in presidential elections.

Iowa’s caucuses have been a source of frustration and excitement since the decision of the Iowa Democratic Party to move its 1972 caucus date, leapfrogging over New Hampshire’s primary to become the first candidate test of the presidential campaign.

In 1976, Jimmy Carter used the caucuses to boost his campaign to national recognition. Even though he came in second, the media attention he received in Iowa helped his campaign in reaching the presidency. Since then, the Iowa caucuses have become an institution, with every competitive presidential candidate making rounds through the state to gain notoriety ahead of Election Day.

It’s been debated that in 2008, presidential candidate Barack Obama would not have won the presidency without winning the Iowa caucuses, having outspent his competitors by almost double in the state.

“On the day of the Iowa caucus, my faith in the American people was vindicated. What you started here has swept across the nation,” he said in a Des Moines campaign speech on Oct. 31, 2008. “So, the people of Iowa, I will always be grateful to all of you!”

Democratic strategist Roger Fisk said candidates will need to recruit volunteers, surrogates and staffers early and campaign extensively using retail, face-to-face contact with Iowa voters to have a shot at 2020.

“These are very, very savvy voters and they’re used to quizzing anyone who wants to present themselves as a potential leader of the free world,” he said. “If the voters in Iowa dig through those layers…and get down to the core of who these people are and it is at odds with who they presented themselves to be at the beginning, that individual will lose.”

Henderson added that voters in Iowa are not easily fooled.

“Sometimes you can run a candidate purely on TV commercials who is not genuine,” said Henderson. “Here in Iowa, we have a way of figuring that out pretty quickly.”

Goldford said Iowa does not take its role in the presidential primary lightly. He said the state recognizes Democrats need a generational shift to new and younger leaders if they want to win back the White House.

“The lack of an obvious next in line or potential Democratic candidate means that every Democrat with aspirations believes he or she has the chance to make a go of it,” he said. “So, you have a whole host of potential Democratic candidates trying to decide whether they have enough potential support here to make the effort worthwhile.”

Whoever the Democratic Party picks as its nominee, he said that person needs to be strong enough of a candidate to compete with President Trump.

“In this case, you have an incumbent president whose instant reaction is when attacked, you strike back doubly hard,” he said. “So, the Democrats have to find someone, if they’re going to be competitive, capable of standing in the ring with someone like that.”