Pete Buttigieg, the 37-year-old South Bend mayor and Democratic 2020 presidential candidate, has seen a recent surge in polling in early-voting primary states. But with his candidacy apparently gaining ground, activists and campaigners on the left are increasingly sounding the alarm over his progressive bona fides.

Buttigieg, considered a moderate within the crowded field, has made clear on the debate stage that he would not pursue the major structural reforms advocated by his fellow candidates Elizabeth Warren and Bernie Sanders, whose platforms are considered more left-leaning.

Activists have accused Buttigieg of being overly reliant on wealthy donors. They also say he is insufficiently committed to racial justice, noting his lack of support among black voters. And they worry that his past – particularly the three years he spent right out of Harvard and Oxford universities working for McKinsey & Company, the world’s largest management consultancy firm – show his interests are aligned with big business.

Nate Levin-Aspenson, co-founder of the South Bend chapter of Indivisible, a grassroots liberal group that sprang up in response to Donald Trump’s election, calls Buttigieg’s decision to work for the firm “mercenary”.

“What people do with their time and talent is a demonstration of their values. And Pete, for all his baggage, is objectively a smart man with a fair amount of talent, and right out of school he chose to go and work for McKinsey. And that says something,” said Levin-Aspenson. “It’s kind of a mercenary thing to do that doesn’t reflect any personal priority beyond career advancement.”

Pete Buttigieg at his Harvard graduation on 10 June 2004 in Cambridge, Massachusetts. After a subsequent stint at Oxford University he went to work for McKinsey. Photograph: AP

Secrecy surrounds much of the work of McKinsey, seen by many as the gold standard in management consulting. While it has worked with Fortune 500 companies, it has also been accused by the Massachusetts attorney general of fanning the flames of the opioid epidemic by advising Purdue Pharma on how to “turbocharge” sales of the drug, and it has worked to expand the influence of authoritarian regimes such as Russia and Saudi Arabia. In his book The Firm: The Story of McKinsey and Its Secret Influence on American Business, Duff McDonald wrote that McKinsey “may be the single greatest legitimizer of mass layoffs”.

Buttigieg has said he was unaware of McKinsey’s work for Purdue Pharma at the time and that its role advising the company was “upsetting”. But for his critics, the details of Buttigieg’s McKinsey work that have been released – including that he studied grocery pricing, and that he traveled to Afghanistan and Iraq as part of his McKinsey work – invite questions.

Buttigieg cannot disclose what projects he worked on while at McKinsey from June 2007 to March 2010 because of confidentiality agreements. His campaign has contacted McKinsey to clarify what the NDA encompasses, and again separately about whether he might be released from it.

Buttigieg’s campaign notes that he has recently released his tax returns from his time at McKinsey and is working with the firm to release even more information about his time there. A spokesman said that Buttigieg has often noted that he disagrees with many of the decisions made by McKinsey and pointed to sections of the candidate’s book, Shortest Way Home, including one where he talks about the moment he realized working for corporate clients “could not be his career for very long” because although “I liked my clients and my colleagues, delivering for them could not furnish that deep level of purpose that I craved”.

Asked recently about his time at McKinsey, Buttigieg responded similarly. “I think it’s important that I have private-sector experience,” he told reporters on his Iowa campaign bus in early November. “Certainly my professional experience, for the most part, has been public service, so I’m happy to speak to it because I think it is important to have a president who understands how things work in the business world. But obviously I hadn’t been in it for very long before I decided to walk away from it for public service.”

Pete Buttigieg has a beer on the bus with reporters after a campaign event in Decorah, Iowa, on 2 November. ‘I think it’s important that I have private sector experience,’ he said. Photograph: Gary He/EPA

Buttigieg’s consulting background contrasts with other frontrunners in the race for the Democratic presidential nomination, such as Warren, who came from academia, and Sanders and the former vice-president Joe Biden, both career politicians. Last month the California senator and former candidate Kamala Harris, who faced her own criticisms from the party’s progressive wing as she positioned herself to compete with Buttigieg for centrist voters wary of a weak Biden candidacy, also sought to draw a contrast between his background and hers.

“For my entire career, I’ve only had one client: The People,” she tweeted following news Buttigieg would release tax returns from his time at McKinsey.

Buttigieg has previously said his time at the firm helped him more fully realize a desire to work in public service. But critics say he remains a business-first candidate. At a time when the party is increasingly hostile to corporate actors, for instance, Buttigieg has continued to oppose the Medicare-for-All-style system favored by Sanders and Warren, which would replace the privately owned healthcare system with a government-run one.

“The fundamental question in politics is whose side are you on, who are you fighting for?” said Adam Green of the Progressive Change Campaign Committee, which has endorsed Warren for president. “Working at McKinsey would be fine if he was now challenging corporate interests like big tech and big insurance companies. But the fact that he’s adopting policies that help them and, in the case of Medicare for All, using industry talking points to attack other Democrats – that’s what makes people ask: is there something about his past work that’s baked in?”

When it’s been politically advantageous to him, Buttigieg has talked up his work at McKinsey. As a candidate for state treasurer of Indiana in 2010, for instance, he told a forum sponsored by Tea Party-aligned groups that he knew how to stabilize economies. And after he lost that election, his McKinsey background and friendly relationship with the business community helped him when he went back home to run for mayor and became the first mayoral candidate the local Chamber of Commerce had ever endorsed.

Pete Buttigieg talks with his leftwing rival Senator Elizabeth Warren during the Democratic presidential debate in Atlanta, Georgia, on 20 November. Photograph: Alex Wong/Getty Images

Nelini Stamp, national organizing director for the Working Families party, which has also endorsed Warren, feels Buttigieg hasn’t done enough to reconcile that past self with his present candidacy. “It’s one thing to say, ‘This is what I did and it was a job and I’ve seen the wrongdoings,’ like when Senator Warren explains why she was a Republican and how she changed, because she actually studied bankruptcy and realized it was not on the individuals, it was on the corporations,” Stamp said.

“If you’re going to distance yourself from it,” she added of his McKinsey background, “own it and say why.”

Now Buttigieg describes his work for McKinsey as “not something that I think is essential in my story”, and many – including Matt Bennett of the center-left thinktank Third Way – agree.

“Rigid orthodoxy and litmus-test politics are a recipe for losing presidential elections and minority status in Congress,” he said. “Some on the far left claim that anyone in Democratic politics who doesn’t share all of their views is an apostate who is controlled by ‘corporate interests’. But that’s offensive and ridiculous.

“Pete Buttigieg opposes Medicare for All for the same reason that Nancy Pelosi, Sherrod Brown and almost every Democrat who flipped a House seat in 2018 oppose it – it’s bad policy and terrible politics.”