From Pete Buttigieg’s old office in South Bend, Indiana, you could see the hospital where he was born, churches built for Irish and Polish immigrants and a factory that made cabinets for Singer sewing machines. “This was the Silicon Valley of its day,” the then mayor told the Guardian in February 2019.

Nearly four years later, Buttigieg is occupying a loftier perch. As America’s transportation secretary, his framed photograph sits alongside those of Joe Biden and Vice-President Kamala Harris in the lobby of the Department of Transportation. The building is located in Navy Yard, a neighbourhood on the Anacostia River that is home to the Washington Nationals baseball team.

Buttigieg has gone from running a city of 100,000 people to a department whose budget is bigger than the gross domestic product of most countries. “As mayor, of course, I worked on a broad range of issues – anything that happened in the city was my concern,” he recalls in a pre-Christmas interview with the Guardian in Washington.

“But here you work with a daunting scope and scale. The scope ranges from commercial space travel to the oversight of our Merchant Marine Academy, so not just planes, trains and automobiles, but everything in between.”

The meteoric rise helps explain why Buttigieg is widely seen as potential presidential material in 2024, 2028 or beyond. He speaks eight languages, had spells at Harvard, Oxford and McKinsey, became a mayor before he turned 30 and did military service in Afghanistan. He won the Democratic presidential caucuses in Iowa in 2020 but, perhaps more importantly, knew when it was time to step aside so the party could unite around Biden.

Now Biden is 80 and Buttigieg is 40, until his next birthday on 19 January. Some Democrats yearn to see generational change, especially if Republicans nominate Ron DeSantis, the 44-year-old governor of Florida, for president in 2024. The Politico website recently highlighted the activities of his allies in a “dark money” group and political action committee under the provocative headline “Pete’s campaign in waiting”.

But part of Buttigieg’s formidable communication skills is a refusal to take such bait. He insists with AI-worthy precision: “I have my hands more than full with my day job and one job at a time is plenty. And it’s a great job and I have a great boss and I’m proud to be part of this team.”

The day job undeniably offers a lot to chew on. American infrastructure ranked just 13th in the world in 2019, according to the World Economic Forum. This was the nation that erected the tallest and most beautiful skyscrapers, built an interstate highway system and put a man on the moon. But in recent decades there has been a sense of turning inward – of decline and neglect – as Asia and Europe raced ahead with gleaming airports and faster trains.

Where did it all go wrong? One answer is President Ronald Reagan, an arch exponent of laissez-faire capitalism who memorably declared that the nine most terrifying words in the English language are “I’m from the government and I’m here to help.”

Buttigieg, who is unapologetically from the government and here to help, says: “The beginning of the Reagan era brought about a vicious cycle of public trust, where resources were stripped away from the government. It became harder for government to deliver for people and then those policy failures reduced trust in government, which made people more reluctant to trust their taxpayer dollars to government, which meant even fewer resources and even worse results.

“The cycle of disinvestment has been accumulating for essentially my entire lifetime and part of what’s so exciting about this moment is a chance to re-establish public trust by making big investments to get big results to build public confidence in the things we can do together through good public policy and good public investment.”

Biden, openly critical of Reagan’s trickle-down economics, set about changing the paradigm. After long negotiations with Congress, including late-night phone calls and several declarations that the deal was dead, he last year signed a trillion-dollar bipartisan infrastructure law.

The money is being – or will be – spent on rebuilding roads, bridges, ports and airports, upgrading public transit and rail systems, replacing lead pipes to provide clean water, cleaning up pollution, providing high-speed internet, delivering cheaper and cleaner energy – and creating thousands of jobs.

One year in, the administration has announced more than $185bn (£154bn) in funding and more than 6,900 specific projects reaching more than 4,000 communities across the country. This includes 2,800 bridge repair and replacement projects and $3bn for 3,075 airport upgrades.

The legislation handed the former “Mayor Pete” the biggest infusion of cash into the transport sector since the 1950s interstate highways. He understands how much is riding on it. “What’s at stake in this transportation legislation – and the president talks about it this way too – is more than just the nuts and bolts of it,” he says.

“It really is a chance to vindicate the democratic system over some of the systems that are trying to challenge us right now in this century. It sounds a little bit cosmic but that really is part of what is on the table right now with our responsibility to deliver.”

The bipartisan law allowed the White House to crow that while “infrastructure week” was a punchline under President Donald Trump, his successor is delivering an “infrastructure decade”. Buttigieg comments: “As you might imagine, I’m no fan of President Trump. I will say this is the one time I was fooled. I actually thought they were going to do it because he talked about it all the time.

“It would have been good politics and everybody wanted it to happen, it would have benefited the economy, and they still couldn’t get it done. So after four years of chest thumping and big promises without results, this administration knew, this president knew, that it was long past time to do something and it turned out the public appetite was there, the deal space was there.”

Even Republicans who voted against the law, branding it a “socialist wishlist”, are happy to reap the benefits. “It’s hard not to chuckle when I get a letter from some member of Congress, invariably a Republican member of Congress, who declared this legislation to be garbage or wasteful social spending or whatever now saying this is funding that really needs to come to my district for these needs. But at the end of the day, it vindicates our approach.”

Buttigieg want to be “strategically shameless” in putting up signs on active projects to make sure that the law gets the credit it deserves. Infrastructure is not like tax policy where, at the stroke of a pen, people feel results instantly. “I often tell the team: part of what we’re doing is building cathedrals and the nature of cathedrals is the person who celebrates the opening may not have been there when the cornerstone was laid.

“But because we’re doing so much at so many different scales and in so many different places, the truth is there’s a range of projects where we’ve already turned a spade, improvements that are going to be felt very quickly to some of the bigger cathedrals that will be years and years in the making.”

Indeed, Democrats insist that some of the positive effects are being felt already. Senator Mark Warner of Virginia tweeted on 19 December: “Week after week, the infrastructure law is paying dividends. It’s expanding highways like I-64, upgrading airports, fixing crumbling bridges and building new bike paths. It’s revitalizing our communities and making every travel day better.

The law’s provisions to tackle systemic racism have come under attack from Republicans and others on the right. Senator Ted Cruz tweeted with sarcasm: “The roads are racist. We must get rid of roads.” DeSantis remarked: “I heard some stuff, some weird stuff from the secretary of transportation trying to make this about social issues. To me, a road’s a road.”

Buttigieg is ready to have that debate. He often notes that the phrase “on the wrong side of the tracks”, referring to the undesirable part of town, is indicative of how a railway or highway not only connects but also divides. “As I’ve had this conversation around the country, it’s striking how, wherever I am, I can see in the faces nodding when I bring this up that people are visualising their own community’s version of this.

“I talk about this not to go around scolding anybody but precisely because we have the means to do better and that’s why it’s so perplexing to see the resistance to it, because, if you have a choice between having a place become more divided or less divided along racial lines through transportation infrastructure choices, why wouldn’t you want it to be less divided?”

At least $1bn (£831m) will help reconnect cities and neighbourhoods that had been racially segregated or divided by road projects. But the legislation is also about including businesses and workers who have been left out in the past.

“There’s some impressive – and sometimes moving – things taking place in the building trades, for example, that are in many places opening their doors to workers of colour and women who will make great skilled labourers and make good incomes to build their families around, who just never would have this opportunity in the last round of major infrastructure investment in this country.”

Transport contributes more greenhouse gases to the US economy than any other sector; Buttigieg wants it to be part of the climate solution as the infrastructure law promises a national network of electric vehicle chargers. Road accidents kill about 40,000 people a year, comparable with gun violence and far worse than other countries; Buttigieg finds this unacceptable and hopes that self-driving cars might be part of the solution.

The secretary, who speaks in paragraphs more polished than most people write, has been willing to make such arguments on Rupert Murdoch’s conservative Fox News network in a series of appearances that have gone viral. It is the kind of outreach to hostile territory that evokes comparisons with Biden’s spirit of bipartisanship – and fuels talk of a future White House run.

He explains: “There are a lot of people who tune into ideological networks, as viewers in good faith who may never hear our administration’s perspective if we’re not out there. I’m not the only one doing it but I have been surprised to see it become something of a speciality.

“You can’t blame somebody for rejecting our approach if they’ve literally never even heard us defend it, especially when it comes to transportation, where most of what we’re doing is actually broadly well-understood and popular but we’ve got to remind people of that.

“It can be tough in a space – and Fox is an example – that tends to offer more coverage of some controversial angle around electric vehicles or racial justice than would offer any coverage of the thousands of specific projects that we’re investing in around the country. I’ve got to get out there and tell people. As long as they’ll have me, I’ll keep doing it.”

Buttigieg recently moved from a red state, Indiana, to an increasingly blue one, Michigan, with his husband Chasten and their two young children. On 13 December the couple were on the White House south lawn to watch Biden sign the Respect for Marriage Act, which protects same-sex and interracial marriages under federal law.

The secretary reflects: “To be sitting with Chasten and seeing the president make that into law was really moving and and reassuring. We shouldn’t have to depend on a one-vote margin on the supreme court to have something as important as millions of marriages be protected and I think Congress recognised that, and I think the American public recognised that.”

The shift in public attitudes was illustrated in last month’s midterm elections, where for the first time LGBTQ+ candidates ran for election in all 50 states and the District of Columbia, and where Oregon’s Tina Kotek and Massachusetts’ Maura Healey ensured that the US will have an out lesbian governor for the first time. Buttigieg himself was in demand as a campaign surrogate for various Democratic candidates.

A New York Times article about him in June 2016 was headlined “The First Gay President?” So is America now ready? “I’m sure it’ll happen,” he says. “What we’re seeing right now is the good, the bad and the ugly. The good news is we have this progress on things like marriage and representation in senior leadership. The bad news is it’s coming in a climate of rights being withdrawn at the US supreme court, including potentially more of the hard-won rights of the LGBTQ+ community.

“And the ugly is you see a level of targeting going on for political convenience, in my view, driven by a lot of figures who don’t want to talk about their lack of solutions on other issues, that can really be costly and even physically dangerous for vulnerable communities right now. You can connect the rhetoric we’ve seen, and some of the legislation we’ve seen in state legislatures, with the sometimes violent atmosphere -especially towards transgender youth but across the board for vulnerable people in this community.”

The interview draws to a close in a meeting room where one wall is dominated by the faces of past transportation secretaries in neat rows. Biden’s Rooseveltian ambitions look set to make Buttigieg the most powerful holder of the office yet.

“Good to see you – and different from the 14th floor in South Bend,” he says affably on his way out. “Who knows where I’ll see you next?”