Tesco’s arrival in California was initially greeted with great fanfare, with the retailer being warmly received by local media for trying to break into the middle ground between the state’s cut-price supermarkets and upscale organic stores.

But while the company has worked hard on its public image it may have underestimated the influence wielded by the main grocery workers’ union, which has the backing of Democratic presidential candidates Barack Obama and Hillary Clinton.

The United Food and Commercial Workers union represents almost 90,000 people in southern California. It is also a member of the Alliance for Healthy and Responsible Grocery Stores, a coalition of community groups, faith-based organisations and unions that has been one of Tesco’s most vocal critics, accusing the group of arrogance and being out of touch with local communities.

The alliance was formed following a long strike by California grocery workers in 2003 and has since succeeded in blocking a proposed Wal-Mart “super-centre” in Inglewood, a city in Los Angles county.

Its members say they want retailers to engage with communities by following through on promises to offer healthcare, good wages and worker training programmes for local employees. The group has requested several meetings with Tesco, which have all been rejected.

Mike Shimpock, speaking for the United Food and Commercial Workers, a member of the alliance, said Tesco had failed to appreciate the close ties between consumers and grocery workers.

“It’s unprecedented that Tesco would refuse to meet with us,” he said. “Barack Obama and Hillary Clinton have both written to Tesco calling on them to meet with us.”

Elliot Petty, retail policy analyst with Los Angeles Alliance for a New Economy, said Tesco’s struggles in southern California were closely related to its “failure to engage” with the group.

“Tesco refused to work with our broad-based coalition and, by extension, they have refused to engage with the consumers we represent,” he said. “I think that is having a direct impact on their business.”

Mr Shimpock added that the profile of grocery workers in southern California was “extraordinarily high . . . in 2003, consumers refused to cross picket lines. People here understand that grocery jobs are some of the last good working-class jobs in southern California.”