(Adds additional Trump tweet, Elizabeth Warren statement)

By David Brunnstrom

WASHINGTON, July 27 (Reuters) - President Donald Trump hit out at a prominent African-American critic on Saturday, calling him a "brutal bully" who should concentrate on cleaning up his "disgusting, rat and rodent infested" Baltimore district rather than criticizing the work of U.S. immigration officers on the Mexican border.

Trump's target in a series of early morning tweets was U.S. Representative Elijah Cummings, Democratic chairman of the House of Representatives Oversight Committee, who has called Trump a racist and sharply criticized his immigration policies.

Democratic House Speaker Nancy Pelosi responded to Trump by calling Cummings a champion for civil rights and economic justice and added in a tweet: "We all reject racist attacks against him and support his steadfast leadership."

On Thursday, the committee voted 23-16 along party lines to allow Cummings to issue subpoenas to White House officials, including Trump's daughter Ivanka Trump and son-in-law Jared Kushner, deepening a probe into potential violations of government record-keeping laws.

Later Saturday, Trump signaled his anger with Cummings' committee efforts, writing in another tweet that the Democrat "spends all of his time trying to hurt innocent people through 'Oversight.'"

Trump, a Republican, took aim at Cummings' criticism of his administration's Mexican border policies.

"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......," Trump tweeted.

"(T)he 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," he said.

In another tweet, Trump questioned why so much money was spent on Cummings' district, which covers the downtown area of the city of Baltimore, about 40 miles (64 km) northeast of Washington, "when it is considered the worst run and most dangerous anywhere in the United States."

"No human being would want to live there. Where is all this money going? How much is stolen? Investigate this corrupt mess immediately," Trump wrote.

'COMPLETELY UNACCEPTABLE'

Last week, Cummings slammed Trump's acting Homeland Security Secretary Kevin McAleenan over conditions of U.S.-Mexico border facilities that house migrants.

"You feel like you're doing a great job, right?" Cummings asked during a congressional hearing. "What does that mean? What does that mean? When a child is sitting in their own feces, can't take a shower? ... None of us would have our children in that position."

Trump's attack follows his broadside two weeks ago against four congresswomen of color, which the Democratic-controlled U.S. House of Representatives condemned as racist.

Cummings responded to Trump on Twitter.

"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," he said.

Cummings also highlighted a letter he co-signed to the president this week expressing "profound disappointment" at his failure to work to lower drug prices.

Baltimore Mayor Bernard Young called Trump's rhetoric "hurtful and dangerous."

"It's completely 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," he tweeted.

Many Democrats running for president came to Cummings' defense on Saturday, with Senator Elizabeth Warren calling Trump's tweets "ugly and racist. They're purposefully designed to turn us against one another."

Cummings represents Maryland's 7th congressional district, which according to 2010 census data was 54.6% black, had a median household income of $51,018, and a college graduation rate of 35.7%.

According to the same census, Maryland was the richest state in the United States, with a median household income of $69,272. Real median household income for the United States as a whole was $49,445 in 2010. (Reporting by David Brunnstrom; additional reporting by David Shepardson Editing by Mary Milliken, Paul Simao and Richard Chang)

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