WASHINGTON – On Saturday morning, President Donald Trump vented on Twitter about a political adversary, Rep. Elijah Cummings, D-Md., and assailed an American city, Baltimore.

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

Trump continued by saying conditions on the border were "clean, efficient & well run, just very crowded," whereas Cummings' district was "a disgusting, rat and rodent infested mess." Trump did not present evidence for this claim about the district.

Cummings, who has been a prominent Trump critic, represents part of Baltimore and its suburbs. According to census data, Cummings' district, the 7th District of Maryland, is 53% black and has an average household income of $60,929.

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Trump also claimed the district was the "worst run and most dangerous" in the United States and called for an investigation into the "corrupt mess" in the district.

Cummings responded later Saturday morning on Twitter, writing "it is my moral duty to fight for my constituents" and asked Trump to act on prescription drug prices.

"Mr. President, I go home to my district daily. Each morning, I wake up, and I go and fight for my neighbors," Cummings said. "It is my constitutional duty to conduct oversight of the Executive Branch. But, it is my moral duty to fight for my constituents."

He continued, "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. 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."

Baltimore Mayor Bernard C. "Jack" Young responded in a statement calling Trump "a disappointment to the people of Baltimore, our country, and to the world."

Trump made similar comments in January 2017 when attacking another black congressman, Rep. John Lewis, D-Ga. Trump had called the civil rights icon's district "in horrible shape and falling apart (not to mention crime infested)" after Lewis said he would be skipping Trump's inauguration.

"Dude, just stop," Rep. Justin Amash, then a Republican and now an independent from Michigan, said of Trump at the time.

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Earlier Saturday morning, "Fox and Friends" aired a segment on Cummings' "dangerous district," in which Republican strategist Kimberly Klacik discussed allegedly downtrodden conditions in West Baltimore. Klacik claimed Trump saw her segment before tweeting.

"The President saw my work. This just made my day," Klacik wrote.

Baltimore had one of the highest homicide rates among major American metropolitan areas, according to FBI data, with 56 homicides for every 100,000 people. The next highest homicide rate among major U.S. cities was Detroit, which had a homicide rate of 40 per 100,000 people.

Only part of Baltimore, though, is in Cummings' district, which also includes suburban and rural areas surrounding the city.

Several members of Congress denounced Trump's remarks after he tweeted them. Rep. Bill Pascrell, D-N.J., called the tweets "revolting racism" and slammed Trump as a "bigot national leader." Rep. Don Beyer, D-Va., said, "If a woman of color serving in Congress talked about American communities the way the President talks about Baltimore or Chicago he would tell her to 'go back' to another country."

Rep. Katherine Clark, D-Mass., quipped, "These are not the words of a patriot."

As chairman of the House Oversight Committee, Cummings has taken on a prominent role in criticizing the Trump administration's policies. On July 18, Cummings' committee held a hearing about the federal government's treatment of migrants in detention during which Cummings sharply critiqued acting Department of Homeland Security Secretary McAleenan.

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"The damage that the Trump administration has inflicted – and is continuing to inflict – will impact these children for the rest of their lives," said Cummings to McAleenan, referring to the detention of migrant children.

Cummings' committee also voted on Thursday to authorize subpoenas for senior White House officials' email and messaging app communications as part of an Oversight Committee investigation into White House officials' use of personal messaging and email accounts to conduct official business. The Oversight Committee is now authorized to issue subpoenas for the communications of White House officials like chief of staff Mick Mulvaney, Trump's adviser and son-in-law Jared Kushner, and Ivanka Trump, also an adviser and the president's daughter.

On July 16, the House of Representatives voted to formally condemn several of Trump's tweets as racist. Trump had attacked four progressive House freshmen colloquially known as the "Squad" and told them on Twitter to "go back" where they came from. Four Republicans and one independent, Justin Amash, joined all Democrats in voting to condemn the remarks.