“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.
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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 tellsVogue: “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 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.)
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.
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.
He adds: “It is bizarre. It makes absolutely no sense except for tradition, and sometimes tradition doesn’t get us anywhere.”