Donald Trump has launched an unprovoked attack on beloved black civil rights leader Al Sharpton after days of repeated racist insults directed at Baltimore and its congressman of colour.

The president tweeted a number of insults towards Mr Sharpton, all of which were posted without accompanying evidence, prompting a public spat between the pair.

Mr Trump’s posts on Monday morning represent his latest in a series of attack directed against people of colour in recent weeks.

In addition to his series of racist posts about Baltimore and its congressman Elijah Cummings, the tweet about Mr Sharpton was immediately followed by another attack on “the squad”, a group of congresswomen of colour who he has repeatedly targeted with racist rhetoric.

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In the new post, the president said he had known Mr Sharpton for 25 years and had been friends with him, before labelling him a “troublemaker” and saying he “hates whites and cops”, while providing evidence for neither claim.

Mr Trump’s attacks had been in response to a tweet by Mr Sharpton, who said he was “headed to Baltimore” in the wake of the president’s attacks. “Long day but can’t stop,” he wrote, without making any explicit reference to the storm over Mr Trump’s comments on the Maryland city.

The president then quoted that tweet and launched a volley of insults against Mr Sharpton.

Soon after the president posted that tweet, Mr Sharpton shared a picture of him with Mr Trump in 2006, in which he said the future president was praising him.

After Mr Sharpton posted the photo, Mr Trump then posted yet another tweet, once again making references to having done “favours” for the civil rights leader, again without providing evidence or examples.

“Al Sharpton would always ask me to go to his events,” he wrote in a follow-up post, after Mr Sharpton had posted the picture.

”He would say, ‘it’s a personal favor to me.’ Seldom, but sometimes, I would go. It was fine. He came to my office in [Trump Tower] during the presidential campaign to apologize for the way he was talking about me. Just a conman at work!”

The war of words between the two men comes amid continuing criticism over Mr Trump for his remarks about Baltimore. The president referred to Mr Cummings’ majority black Baltimore district as a “disgusting, rat and rodent infested mess” where “no human being would want to live”.

The president has repeatedly used the term ”infested”, but seemingly only ever to describe places where people of colour live.

He has refused to acknowledge the racist rhetoric in the tweets, instead baselessly calling the black lawmaker racist.

Speaking in television interviews, acting White House chief of staff Mick Mulvaney said Mr Trump was reacting in frustration to the Democrats’ unrelenting investigations and talk of impeachment.

He said Mr Trump swung hard at Mr Cummings and his Baltimore district because he believed such Capitol Hill critics are neglecting serious problems back home in their zeal to unfairly undermine his presidency.

“I understand that everything that Donald Trump says is offensive to some people,” Mr Mulvaney said.

But he added: “The president is pushing back against what he sees as wrong. It’s how he’s done it in the past, and he’ll continue to do it in the future.”

Mr Mulvaney, a former congressman, said he understood why some people could perceive Trump’s words as racist.

He added that Mr Trump’s words were exaggerated for effect - “Does the president speak hyperbolically? Absolutely” - and that they were intended to draw attention to Democratic-backed investigations of the Republican president and his team in Washington.

“Instead of helping people back home, they’re focusing on scandal in Washington DC, which is the exact opposite of what they said they would do when they ran for election in 2018,” Mulvaney said, pointing at Democrats who now control the House.

He asserted that Mr Trump’s barbs were a reaction to what the president considered to be inaccurate statements by Mr Cummings about conditions in which children are being held in detention at the U.S.-Mexico border.

At a hearing last week, Mr Cummings accused a top administration official of wrongly calling reports of filthy, overcrowded border facilities “unsubstantiated.”

“When the president hears lies like that, he’s going to fight back,” Mr Mulvaney said.

Mr Trump’s tweets on Saturday also suggested that Cummings’ district, which includes Johns Hopkins Hospital, the Social Security Administration and the national headquarters of the NAACP, is “considered the worst run and most dangerous anywhere in the United States.”

Condemnation followed from Democrats over the weekend, including some of the party’s presidential candidates. Statements from a spokesman for Maryland’s Republican governor and from the lieutenant governor defended Cummings’ district and its people.

The president has tried to put racial polarisation at the centre of his appeal to his base of voters, tapping into anxieties about demographic and cultural changes in the nation in the belief that the divided country he leads will simply choose sides over issues such as race.

Mr Mulvaney argued that Mr Trump would criticise any lawmaker, no matter the person’s race, in a similar way if he felt that individual spoke unfairly about the president’s policies.

He claimed that if Adam Schiff, the California Democrat who leads the House Intelligence Committee, had made the same remarks as Mr Cummings, Mr Trump would have pushed back.

“It has zero to do with the fact that Adam is Jewish and everything to do with Adam would just be wrong if he were saying that,” Mr Mulvaney said. “This is what the president does. He fights and he’s not wrong to do so.”

To Mr Mulvaney, Mr Trump was “right to raise” the challenges faced in Cummings’ district at the same time that Cummings and other Democrats are “chasing down” the Russia investigation undertaken by Robert Mueller and pursuing “this bizarre impeachment crusade.”

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Mr Cummings is leading multiple investigations of the president’s governmental dealings. In his direct response to Trump on Twitter, Mr Cummings said: “Mr President, I go home to my district daily. Each morning, I wake up, and I go and fight for my neighbors. It is my constitutional duty to conduct oversight of the Executive Branch. But, it is my moral duty to fight for my constituents.”

Mr Cummings has also drawn the president’s ire for investigations touching on his family members serving in the White House. His committee voted along party lines Thursday to authorise subpoenas for personal emails and texts used for official business by top White House aides, including Ivanka Trump and her husband, Jared Kushner.

Democratic presidential candidate Julian Castro said Mr Trump was engaging in “racial priming.”

“Using this language and taking actions to try and get people to move into their camps by racial and ethnic identity. That’s how he thinks he won in 2016 and that’s how he thinks he’s going to win in 2020,” Castro said.

Earlier this month, Mr Trump drew bipartisan condemnation following his call for Democratic representatives Ilhan Omar of Minnesota, Alexandria Ocasio-Cortez of New York, Ayanna Pressley of Massachusetts and Rashida Tlaib of Michigan to get out of the US “right now.”

He said that if the lawmakers “hate our country,” they can go back to their “broken and crime-infested” countries.

All four lawmakers of colour are American citizens and three of the four were born in the US The House later voted largely along party lines to condemn his “racist comments.”

Mr Mulvaney was interviewed on “Fox News Sunday” and CBS’ “Face the Nation,” where Castro also appeared.