Democrat leader Nancy Pelosi accuses Trump’s of 'racist attacks' for branding Baltimore 'rodent infested mess'

Trump's outburst came in a series of sharply worded tweets aimed at Cummings - a high-profile critic of Trump's administration whose district covers much of Baltimore.

"Cumming (sic) District is a disgusting, rat and rodent infested mess," the president wrote, calling it "the worst run and most dangerous anywhere in the United States".

"No human being would want to live there," he said - in an attack ostensibly provoked by Cummings' criticism of the harsh conditions facing would-be asylum seekers at the US-Mexico border.

The morning diatribe ignited a storm of criticism, less than two weeks after the House of Representatives condemned Trump for "racist" comments targeting four first-term Democratic congresswomen who are ethnic minorities.

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Former vice-President Joe Biden - the Democratic frontrunner to challenge Trump in 2020 - called out the president directly on Twitter.

"It is despicable for you to attack him and the people of Baltimore this way," Biden wrote.

"Once again you have proved yourself unfit to hold the office. A President is supposed to lift this nation up. Not tear it down."

There was similar condemnation from half a dozen White House candidates including Cory Booker, who tweeted footage of a black CNN anchor and Baltimore native who broke down on air while reacting to the attack on his hometown.

'Disgraceful'

Democratic 2020 contender Kamala Harris, who like Booker is black, said she was "proud" to have her campaign headquarters in Cummings' district and calling Trump's attack "disgraceful".

The city's Democratic Mayor Bernard "Jack" Young  - also black - rejected Trump's rhetoric, calling it "hurtful and dangerous".

"It was a total insult," he told reporters. "We're not going to ignore anyone degrading Baltimore city and its effective leadership, no one."

A historic port city of 600,000 people, Baltimore presents a mixed picture, with both handsome and affluent neighborhoods and poverty-stricken districts. It has one of the country's highest murder rates. 

Cummings' district is more than 50 percent black - and the city of Baltimore as a whole more than 60 percent.

Cummings himself tweeted: "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."

As chairman of the House Oversight Committee, Cummings - one of the most prominent African Americans in Congress - has launched investigations into Trump administration policies, including reports of poor treatment at migrant detention centers.

"Rep. Elijah Cummings has been a brutal bully, shouting and screaming at the great men & women of Border Patrol about conditions at the Southern Border, when actually his Baltimore district is FAR WORSE and more dangerous," Trump charged.

He attacked Cummings again Saturday evening after spending the day at Trump National Golf Club outside Washington.

"Elijah Cummings spends all of his time trying to hurt innocent people through 'Oversight.' He does NOTHING for his very poor, very dangerous and very badly run district!," the president tweeted, adding the hashtag #BlacksForTrump2020.

The broadside was reminiscent of Trump's recent, racially-charged onslaught against the four young Democratic women lawmakers, who he said should "go back" to the "crime infested" places they came from. In fact, three were born in the US and all are American citizens.

In an op-ed piece for The Washington Post late Friday, 148 African Americans who served under former president Barack Obama pledged their support for the four "as well as all those currently under attack by President Trump".

Trump denies accusations of racism - and has made a point of pushing for the release of US rapper A$AP Rocky, held in Sweden on assault charges, in an apparent effort to curb his unpopularity with black voters.

But his calculated targeting of the "Squad" - as the four are known - was widely seen as a bid to galvanize his mostly white electoral base as he gears up for next year's reelection battle.