President Donald Trump on Saturday denigrated a majority-black district represented by a congressional nemesis as a "disgusting, rat and rodent infested mess," broadening a campaign against prominent critics of his administration that has exacerbated racial tensions. Mr Trump's outburst came in a series of sharply worded tweets aimed at Democratic Representative Elijah Cummings – a high-profile critic of Mr Trump's administration whose district covers much of Baltimore. "Cumming 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 Mr Cummings's 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 Mr Trump for "racist" comments against four first-term Democratic congresswomen who are ethnic minorities. The top Democrat in Congress, Nancy Pelosi, accused Mr Trump of a "racist" attack on a "champion... of civil rights and economic justice, a beloved leader in Baltimore, and deeply valued colleague." "We all reject racist attacks against him," wrote Ms Pelosi – who was born in Baltimore and whose father served as mayor of the city. Former vice president Joe Biden – the Democratic frontrunner to challenge Mr 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," Mr 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." Democratic 2020 contender Kamala Harris, who like Booker is black, said she was "proud" to have her campaign headquarters in Cummings's district and called Mr Trump's attack "disgraceful". Baltimore Mayor Bernard C "Jack" Young fired back at Mr Trump after his attacks, saying he is a "disappointment to the people of Baltimore, our country, and to the world." "It's completely unacceptable for the political leader of our country to denigrate a vibrant American City like Baltimore, and to viciously attack US Representative Elijah Cummings a patriot and a hero," Mr Young said in a statement. Mr Cummings's district is more than 50 per cent black – and the city of Baltimore as a whole more than 60 per cent. Mr 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 centres. Mr Trump called Mr Cummings a "brutal bully" after his public tongue-lashing of top Homeland Security officials over conditions for migrants detained along the southern border. "As proven last week during a Congressional tour, the Border is clean, efficient & well run, just very crowded," Trump tweeted. "Cumming District is a disgusting, rat and rodent infested mess." 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 Mr Trump's recent, racially charged onslaught against the four young Democratic women politicians, 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. Mr 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.