Donald Trump attacked Mike Bloomberg at a rural water policy event in California, saying the former New York City mayor and Democratic presidential candidate "hates the farmer."

The president's attack on Mr Bloomberg, who is surging in the polls as he makes his Democratic debate stage debut later on Wednesday night, was merely his latest on a possible general election foe he derisively calls "Mini-Mike," a dig at Mr Bloomberg's height.

Like other Democratic candidates, the former Big Apple mayor leads the incumbent in hypothetical head-to-head general election polls – by some margins larger than a poll's margin of error.

The president interrupted the event in Bakersfield to tell the crowd that "Mini Mike" harbors "disrespect of the farmer."

He appeared to be referring to comments Mr Bloomberg made in November 2016, shortly after Mr Trump was elected.

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"If you think about it, the agrarian society lasted 3,000 years, and we can teach processes. I can teach anybody – even people in this room, so no offense intended – to be a farmer. It's a process. You dig a hole, you put a seed in, you put dirt on top, you add water, up comes corn. You can learn that.

"Then you had 300 years of the industrial society. You put the piece of metal on the lathe, you turn the crank in direction of arrow and you can have a job. And we created a lot of jobs. At one point, 98% of the world worked in agriculture. Today it's 2% of the United States," he said to an audience of students and others at the Saïd Business School at the University of Oxford in the UK.

"Now comes the information economy and the information economy is fundamentally different because it's built around replacing people with technology, and the skill sets that you have to learn are how to think and analyze," Mr Bloomberg added.

"And that is a whole degree level different. You have to have a different skill set. You have to have a lot more gray matter. It's not clear the teachers can teach or the students can learn. And so the challenge of society is to find jobs for these people."

A Bloomberg campaign spokesman said in an email that the former mayor "wasn't talking about today's farmers at all, and Team Trump is misleading Americans because Donald Trump's erratic policies have devastated American farms, including a 20 per cent increase in US farm bankruptcies last year."

"Donald Trump inherited his wealth yet bankrupted businesses in cities around the world. Now he's hurting American farms, and he knows that Mike Bloomberg has the skills to fix the problem," said the spokesman, Stu Loeser.

Notably, Mr Bloomberg's top Democratic opponent for the party's 2020 nomination, Vermont Senator Bernie Sanders, also has criticised the remarks.

The nonpartisan fact-checking organisation FactCheck.org analyzed Mr Bloomberg's remarks and others on the topic and opted against issuing a judgement on whether he indeed opposes farmers.

"But we leave it up to voters to decide if Bloomberg's words – in their full context – show he 'really hates regular hardworking Americans,' as Donald Trump Jr tweeted, or were 'insulting the middle class and working class,' as People for Bernie claims."