Democratic House speaker Nancy Pelosi has accused US President Donald Trump of "racist attacks" in tweets about an African American lawmaker.
Mr Trump attacked Democratic Representative Elijah Cummings and his Maryland district on Twitter.
The president described Mr Cummings' majority-black district in Baltimore as a "rodent-infested mess".
Mr Cummings was a "bully", Mr Trump wrote, for criticising the treatment of migrants at the US-Mexico border.
As chairman of the House Oversight Committee, Mr Cummings has instigated a series of investigations into the Trump administration's policies, including its handling of migrants at detention centres.
Ms Pelosi led Democratic legislators in defending Mr Cummings and condemning Mr Trump's tweets.
The tweets, Ms Pelosi wrote, were "racist attacks" on Mr Cummings, whose district's population is more than 50 percent black, according to US census data.
.@RepCummings is a champion in the Congress and the country for civil rights and economic justice, a beloved leader in Baltimore, and deeply valued colleague. We all reject racist attacks against him and support his steadfast leadership. #ElijahCummingsIsAPatriot https://t.co/2LG8AuQrHh— Nancy Pelosi (@SpeakerPelosi) July 27, 2019
A prolific critic of Mr Trump, Mr Cummings responded in kind, tweeting: "It is my constitutional duty to conduct oversight of the Executive Branch. But, it is my moral duty to fight for my constituents."
Baltimore's mayor, Bernard "Jack" Young, called Mr Trump "a disappointment to the people" of his city, and to "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 tweeted.
Other Democrats, including presidential candidates Joe Biden and Kamala Harris, have derided Mr Trump's tweets.
The response from Republican Party representatives has been muted so far.
The episode has echoes of the racially-charged rhetoric Mr Trump used in tweets lambasting four Democratic congresswomen of colour earlier this month.
Mr Trump suggested the congresswomen - Alexandria Ocasio-Cortez, Rashida Tlaib and Ayanna Pressley, and Ilhan Omar, all of whom are US citizens - should "go back" to the "crime infested" places they came from.
What sparked Mr Trump's tweets?
Last week, Mr Cummings lashed out at acting Homeland Security Secretary Kevin McAleenan and conditions at migrant detention centres at a congressional hearing.
In a heated exchange with Mr McAleenan, Mr Cummings demanded "improvement" at border facilities.
Mr Cummings, who represents Maryland's 7th congressional district, suggested the Trump administration had "an empathy deficit" in its handling of migrants.
Posting to Twitter on Saturday, Mr Trump appeared outraged at Mr Cumming's comments.
....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— Donald J. Trump (@realDonaldTrump) July 27, 2019
The tweets also come days after a Democratic-led congressional committee voted to subpoena the private communications of senior White House officials.
Mr Cummings accused the Trump administration of not keeping communications records in compliance with federal law.
The committee's investigation is one of several being pursued by House Democrats into the president and his administration.