She wrote on Twitter that “it is so sad” to interpret turning “the light currently shining on #WestBaltimore into a race issue,” adding that Mr. Cummings “could help get his district in order.”

Undaunted by the criticism, Mr. Trump decided to again go after Mr. Cummings, the chairman of the House Oversight and Reform Committee, and explicitly referred to race by asserting that African-Americans support him. “Elijah Cummings spends all of his time trying to hurt innocent people through ‘Oversight,’” the president tweeted late in the day. “He does NOTHING for his very poor, very dangerous and very badly run district!” He then added: “#BlacksForTrump2020.”

Mr. Cummings’s district is 53 percent African-American, according to the census, and includes much of Baltimore as well as vast suburban stretches. Baltimore has struggled with crime in recent years, recording more murders in 2017 than any other city of at least 500,000 residents — and more even than New York, a vastly larger city.

Mr. Trump has a history of characterizing predominantly black areas or African nations in terms like “filthy.” During his 2016 campaign, he portrayed black communities as unremittingly grim. “You’re living in your poverty, your schools are no good, you have no jobs,” he said at one rally, arguing that African-Americans should give him a chance. “What the hell do you have to lose?”

When Representative John Lewis, a Democrat from Georgia considered a hero of the civil rights era, said he would not attend Mr. Trump’s inauguration in January 2017, the incoming president said that the congressman “should spend more time on fixing and helping his district, which is in horrible shape and falling apart.” Mr. Trump disparaged Haiti and African nations last year using vulgar language, asking why the United States should want immigrants from them.

Mr. Trump has denied charges that he is racist, citing in his defense the low unemployment rates for Hispanics and African-Americans on his watch, among other things. In recent days, he has also made a point of pressuring Sweden to release the rapper ASAP Rocky, who was charged with assault there, saying, “Sweden has let our African American Community down in the United States.”

Mr. Cummings has been one of the president’s most persistent critics in Congress. Only two days ago, he was authorized by his oversight committee to subpoena work-related text and emails sent on personal accounts by White House officials, including Mr. Trump’s daughter and son-in-law.