In a series of tweets on Saturday morning, President Trump slammed Rep. Elijah Cummings for his criticism of treatment of migrants at the southern border, claiming that Cummings' congressional district was one of the "worst run and most dangerous anywhere in the United States."

"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. His district is considered the Worst in the USA," Mr. Trump said in the first of several tweets.

"As proven last week during a Congressional tour, the Border is clean, efficient & well run, just very crowded. Cumming District is a disgusting, rat and rodent infested mess. If he spent more time in Baltimore, maybe he could help clean up this very dangerous & filthy place," Mr. Trump continued. He threatened to investigate the federal funds sent to Cummings' district, and said that "no human being would want to live there."

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Mr. Trump doubled down hours later, writing "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!" He included a video from Terrance K. Williams, a black conservative comedian.

While Cummings' district encompasses some of Baltimore, it also includes some of the wealthier areas in Maryland. His district has an above-average median income compared to the rest of the U.S. His district is also over 50% black, according to Census data. However, Baltimore has struggled with crime in recent years. In 2017, Baltimore had the highest homicide rate among the country's 50 largest cities.

Mr. Trump's tweets quickly stirred outrage online, with many questioning why he would assume the black representative would preside over a squalid district. It is unclear what specifically triggered the attacks on the chairman of the powerful House Oversight and Reform Committee. However, Matthew Gertz of Media Matters reported that a segment criticizing Cummings and his district aired earlier Saturday morning on "Fox & Friends," one of Mr. Trump's preferred cable news shows.

In a segment which quickly went viral, CNN host Victor Blackwell, a native of Baltimore, noted that Mr. Trump often invokes infestation when referring to minority lawmakers, such as when he tweeted that four progressive congresswomen should "go back" to the "crime infested places from which they came."

"The president says about Congressman Cummings' district that no human would want to live there. You know who did, Mr. President? I did," Blackwell said, choking up with emotion. "There are challenges, no doubt. But people are proud of their community."

"I don't want to sound self-righteous, but people get up and go to work there, they care for their families there, they love their children who pledge allegiance to the flag just like people who live in districts of congressmen who support you, sir. They are Americans too," Blackwell said.

In a statement, Baltimore Mayor Bernard C. "Jack" Young said it was "unacceptable for the political leader of our country to denigrate a vibrant American City like Baltimore, and to viciously attack U.S. Representative Elijah Cummings a patriot and a hero."

Cummings responded to Mr. Trump on Twitter in a series of tweets, saying that "Each morning, I wake up, and I go and fight for my neighbors." Cummings added that he held a hearing on skyrocketing drug prices this week.

"Mr. President, we can address this together. Two years ago, I went to the White House to ask you to endorse my bill to let the government negotiate directly for lower drug prices," Cummings wrote. "You told me then that you supported the legislation and that you would work with me to make it happen. I took you at your word," he said in his final tweet, linking to a 2017 letter by House Democrats expressing disappointment that Mr. Trump did not work with them on lowering prescription drug prices.

In a statement to CBS News, Cummings said he "will not be distracted" by Mr. Trump's tweets.

"My tweets are all I have to say. I will not be distracted from doing everything I can to uplift the lives of the people in Baltimore, the nation, and the world. That is and will be my sole mission until the day I die," Cummings said.

Cummings has been a vocal critic of treatment of migrants at detention facilities at the border. His committee held a hearing on these facilities earlier this month. In his opening statement at that hearing, Cummings noted the reports of squalid conditions for detained migrant children, and urged committee members to ask themselves, "Would you allow this for your own child?"

His committee also authorized subpoenas for personal emails and texts used for official business by top White House advisers, including Mr. Trump's daughter and son-in-law, Ivanka Trump and Jared Kushner.