“We can’t go around thanking black women for powering Democrats to victory all over the country, and then at the same time hold our first caucus and our first primary in states that have almost no African-Americans,” he says. “I believe we need to change the order of the states.”

In Iowa, comments such as those by Barack Obama’s one-time housing secretary count as heresy. For five decades, this lightly populated, rural state has basked in its status in being the first in the nation to vote, something that empowers it with a hugely outsized influence over how the parties pick a candidate.

But things may be changing. Critics such as 45-year-old Mr Castro, one of the 20 or so candidates seeking the Democratic Party’s nomination, are calling out Iowa for being too white, too old, and concerned about agriculture and wheat prices. In short, it is no longer reflective of America’s politics, even if it once was.

He tells Vogue: “We’re right to call Republicans out when they suppress the votes of African-Americans or Latinos, but we’ve also got to recognise that this 50-year-old process was created during a time when minority voices had zero power in the party.”

The Democrat challengers to Trump in 2020 Show all 25 1 /25 The Democrat challengers to Trump in 2020 The Democrat challengers to Trump in 2020 Bernie Sanders The Vermont senator has launched a second bid for president after losing out to Hilary Clinton in the 2016 Democratic primaries. He is running on a similar platform of democratic socialist reform Getty The Democrat challengers to Trump in 2020 Joe Biden The former vice president recently faced scrutiny for inappropriate touching of women, but was thought to deal with the criticism well and has since maintained a front runner status in national polling EPA The Democrat challengers to Trump in 2020 Elizabeth Warren The Massachusetts senator is a progressive Democrat, and a major supporter of regulating Wall Street Reuters The Democrat challengers to Trump in 2020 Amy Klobuchar Klobuchar is a Minnesota senator who earned praise for her contribution to the Brett Kavanaugh hearings Getty The Democrat challengers to Trump in 2020 Michael Bloomberg Michael Bloomberg, a late addition to the 2020 race, announced his candidacy after months of speculation in November. He has launched a massive ad-buying campaign and issued an apology for the controversial "stop and frisk" programme that adversely impacted minority communities in New York City when he was mayor Getty Images The Democrat challengers to Trump in 2020 Tulsi Gabbard The Hawaii congresswoman announced her candidacy in January, but has faced tough questions on her past comments on LGBT+ rights and her stance on Syrian President Bashar al-Assad Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Pete Buttigieg The centrist Indiana mayor and war veteran would be the first openly LGBT+ president in American history Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Deval Patrick The former Massachusetts governor launched a late 2020 candidacy and received very little reception. With just a few short months until the first voters flock to the polls, the former governor is running as a centrist and believes he can unite the party's various voting blocs AFP/Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Beto O'Rourke The former Texas congressman formally launched his bid for the presidency in March. He ran on a progressive platform, stating that the US is driven by "gross differences in opportunity and outcome" AP The Democrat challengers to Trump in 2020 DROPPED OUT: Kamala Harris The former California attorney general was introduced to the national stage during Jeff Sessions’ testimony. She has endorsed Medicare-for-all and proposed a major tax-credit for the middle class AFP/Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Bill De Blasio The New York mayor announced his bid on 16 May 2019. He emerged in 2013 as a leading voice in the left wing of his party but struggled to build a national profile and has suffered a number of political setbacks in his time as mayor AFP/Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Steve Bullock The Montana governor announced his bid on 14 May. He stated "We need to defeat Donald Trump in 2020 and defeat the corrupt system that lets campaign money drown out the people's voice, so we can finally make good on the promise of a fair shot for everyone." He also highlighted the fact that he won the governor's seat in a red [Republican] state Reuters The Democrat challengers to Trump in 2020 DROPPED OUT: Cory Booker The New Jersey Senator has focused on restoring kindness and civility in American politics throughout his campaign, though he has failed to secure the same level of support and fundraising as several other senators running for the White House in 2020 Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Wayne Messam Mayor of the city of Miramar in the Miami metropolitan area, Wayne Messam said he intended to run on a progressive platform against the "broken" federal government. He favours gun regulations and was a signatory to a letter from some 400 mayors condemning President Trump's withdrawal from the Paris Climate Accord Vice News The Democrat challengers to Trump in 2020 DROPPED OUT: Kirsten Gillibrand The New York Senator formally announced her presidential bid in January, saying that “healthcare should be a right, not a privilege” Getty The Democrat challengers to Trump in 2020 DROPPED OUT: John Delaney The Maryland congressman was the first to launch his bid for presidency, making the announcement in 2017 AP The Democrat challengers to Trump in 2020 DROPPED OUT: Andrew Yang The entrepreneur announced his presidential candidacy by pledging that he would introduce a universal basic income of $1,000 a month to every American over the age of 18 Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Julian Castro The former San Antonio mayor announced his candidacy in January and said that his running has a “special meaning” for the Latino community in the US Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Marianne Williamson The author and spiritual adviser has announced her intention to run for president. She had previously run for congress as an independent in 2014 but was unsuccessful Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Eric Swalwell One of the younger candidates, Swalwell has served on multiple committees in the House of Representatives. He intended to make gun control central to his campaign but dropped out after his team said it was clear there was no path to victory Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Seth Moulton A Massachusetts congressman, Moulton is a former US soldier who is best known for trying to stop Nancy Pelosi from becoming speaker of the house. He dropped out of the race after not polling well in key states Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Jay Inslee Inslee has been governor of Washington since 2013. His bid was centred around climate change AFP/Getty The Democrat challengers to Trump in 2020 DROPPED OUT: John Hickenlooper The former governor of Colorado aimed to sell himself as an effective leader who was open to compromise, but failed to make a splash on the national stage Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Tim Ryan Ohio representative Tim Ryan ran on a campaign that hinged on his working class roots, though his messaging did not appear to resonate with voters Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Tom Steyer Democratic presidential hopeful billionaire and philanthropist Tom Steyer is a longtime Democratic donor AFP/Getty

The issue appears particularly relevant this year. Polls suggest Pete Buttigieg, the white mayor of South Bend, Indiana, is leading in Iowa. Yet the same polls indicate he has little traction in South Carolina, the fourth state to vote, where his polling among African-Americans, who make up a large proportion of voters, is close to zero.

“What people are going to be looking for in a candidate in a largely white state is going to be different to a place like South Carolina or Nevada,” says Aimee Allison, founder of She the People, a group that works to promote the political power of women of colour.

She tells The Independent: “It’s a structural racism question, because Iowa has an outsized voice in trying to winnow the field, and it’s a poor reflection of the multiracial democracy a lot of us are committed to building.”

It is often said that Iowans take very seriously their role as national political screeners. Because candidates are obliged to repeatedly visit here and New Hampshire, the second place to vote, people have an opportunity to scrutinise and meet many of them, and frequently pose questions.

But data suggests the process is far from perfect. Turnout in the 2016 caucus was just 15.7 per cent. This year, observers believe it could match the high of 16.1 per cent from 2008 election, which was the year Mr Obama was among the candidates. (He came first, defeating both John Edwards and Hillary Clinton, by 10 points.)

The process for voting in Iowa also sometimes gets mythologised. Along with Nevada and Wyoming, Iowa is one of just three states to hold a caucus, rather than a primary.

Iowa became the first state to vote after the Democratic Party changed its rules in 1968 to try to make the process more representative, after George McGovern won the nomination without having won a single primary. The Hawkeye State was chosen to go first. (McGovern ended up losing to Richard Nixon.)

Confronting snow: just one of the challenges for candidates campaigning in Iowa (Andrew Buncombe)

Voting is public and in person, and involves self-appointed surrogates for the candidates arguing on their behalf, and votes being tallied up. Critics point out a secret ballot lies at the heart of genuine democracy, and that people can feel pressured if they have to vote with a show of the hand.

The fact there is no postal ballot also discriminates against people who cannot get to the caucus centre, because they are ill, or because they have to work.

“What if I show up to vote, and my boss is there, and she supports somebody else?” says Christina Greer, a professor of political science at New York’s Fordham University.

Iowa stubbornly fights to hang on to its special status, aware its economy also gets a boost from the spending in a political cycle that seems to start earlier and earlier. It is said that Tom Perez, chair of the Democratic National Committee, earned the support of Iowa delegates after vowing it would continue to be first to vote.

Some officials even claim it is representative, even though a 2016 study by the Brookings Institution found New Hampshire and Iowa had the second and fifth smallest minority electorates respectively. (Vermont was the least diverse.) In Iowa, the population is 90 per cent white, which compares with 60 per cent nationally.

Pete Butegieg democratic party candidate interviewed about his chances of winning an election

It also found the two states have electorates that are older and less urban than the nation as a whole.

Some Iowans acknowledge there’s a problem. Charles Richards, a 65-year-old attending an Elizabeth Warren rally in West Des Moines last week, says Iowa is politically “stagnant”. The state’s senior senator, Republican Chuck Grassley, was first elected in 1981.

“I’m not sure we should do the first-in-the-nation thing,” he adds.

This is an issue that most candidates avoid discussing. Ms Warren told a forum in South Carolina: “Are you actually going to ask me to sit here and criticise Iowa and New Hampshire? I’m just a player in the game.”

This week, the senator Amy Klobuchar told voters in Saint Ansgar, a small community close to the border with Minnesota, that she supports changing the way the US elected its president, saying it was time to drop the electoral college system favoured by the drafters of the constitution, and replace it with a simple, popular vote. On five occasions, a candidate has won the popular vote, but lost the electoral college, and with it the presidency, most recently in 2000 and 2016.

Yet the senator suggests she is not so open about replacing Iowa as the first state to vote.

She says: “I think the fact that we have four very different states, including Nevada and South Carolina, as well as Iowa and New Hampshire, is important to our election.”

In a jab at former New York mayor Michael Bloomberg, who has entered the race with a $30m (£23m) expenditure on television adverts but who plans to avoid campaigning in the first four states, she adds: “I don’t want to have someone who can just run $30m in the big states. I think it’s important to get out and talk to the voters.”

Other states have their eye on Iowa’s crown.

Gilberto Hinojosa, chair of Texas’ state Democratic Party, tells The New York Times he has struggled to have candidates commit to attending a planned event in the state that has the second most electoral college votes – 38 – because they were too busy in Iowa and New Hampshire.

“It is not right that we have a caucus in Iowa that makes it difficult for people to vote, and right after that a primary in New Hampshire, in a little tiny state that does not represent the diversity of America,” he says.