A California deli owner who offered a free side to customers declaring "send her back" has found himself in the middle of a nationwide controversy over President Donald Trump's racist suggestion that Representative Ilhan Omar, D-Minn., a naturalised citizen, return to her native Somalia.

On Friday, John Canesa, owner of Canesa's Brooklyn Heros in Clayton, California, posted a short message on Facebook.

It read: "Meatballs made with beef today incase [sic] we offend any of our overly sensitive pork haters!! Free side when you say send her back! #canesasbrooklynheroes."

The post appeared to take a swipe at Muslims in general (because they don't eat pork on religious grounds) and Omar specifically. KTVU, the San Francisco Bay area Fox affiliate, first reported the story.

The post has since been deleted or Canesa has made his Facebook page private. Either way, screen captures of the original message have been making their way across the internet, forcing locals in the East Bay Area community to pick sides in the battle.

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Some have taken to Yelp, which has temporarily blocked people from rating the business, and other social media platforms to express their outrage. Others have called for a boycott of the deli.

But on Tuesday, customers were lining up outside Canesa's to show their support for the business and the owner, despite temperatures that ranged in the upper 90s, according to the San Francisco Chronicle. "It's getting blown way out of proportion," one Clayton resident told the Chronicle. "I know him and he's not a racist."

Tuija Catalano, the foreign-born mayor of Clayton and a practicing attorney, condemned the promotion at Canesa's in a Facebook post of her own.

Canesa's Brooklyn Heros offered a free side dish to diners who repeated Trump's racist trope 'send her back' (Google)

"I am very saddened to have seen a FB post by the owner of local deli this morning. We all have rights to our own political, religious, and other opinions. We all have a right to post about them too.

"However, there is no place in our community for hatred and bigotry. When hateful comments are being promoted as part of a local business, they reflect on our community's reputation.

"As an elected official, who is also a woman and foreign-born, I personally find a comment about sending anyone back over their political opinions unacceptable."

The episode at Canesa's is further evidence that Trump's racist tweets from 14 July have taken on a life of their own.

In the thread, the president suggested the "'Progressive' Democratic Congresswomen" – presumably the four minority lawmakers known as the Squad: Reps. Omar, Alexandria Ocasio-Cortez of New York, Ayanna Pressley of Massachusetts and Rashida Tlaib of Michigan – "go back and help fix the totally broken and crime infested places from which they came."

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But of the four congresswomen, only one, Omar, was born outside the United States. She was born in Somalia and came to the United States as a refugee.

Even as Democrats and some Republicans condemned Trump's tweets, the president doubled down on his remarks. On Monday, he tweeted that the four minority lawmakers were "very Racist" and "not very smart." On Tuesday, the House voted to condemn Trump's comments aimed at the congresswomen.

At a campaign stop last week in Greenville, North Carolina, Trump supporters turned the president's latest controversy into a rallying cry. Thousands chanted "send her back," apparently a reference to deporting Omar back to Somalia.

Trump, according to news reports, paused his speech as the chants grew louder. "He listened for 13 seconds, as the stray shouts stretched into a uniform roar," reporter Meagan Flynn wrote.

"Some raised campaign signs or fists to the rhythm of the crowd as they yelled. Some only murmured along. A few were children, following the lead of the adults sitting in front-row seats behind President Trump."

Now the residents of a small California town have been dragged into the controversy. Contra Costa County, where Clayton is located, seems an unlikely battleground over immigrants in the United States.

Although nearly 500,000 of the county's residents are white, more than 435,000 are Hispanic and Asian. What's more, in the 2016 presidential elections, nearly 68 per cent of the county's vote went to Hillary Clinton. Trump received only 25 per cent of the votes.

Canesa could not be reached for comment.