Democratic House speaker Nancy Pelosi has accused US President Donald Trump of "racist attacks" in his tweets about an African-American lawmaker.
Mr Trump attacked Democratic Rep Elijah Cummings and his Maryland district on Twitter.
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 Trump's tweets.
The tweets, Ms Pelosi wrote, were "racist attacks" on Mr Cummings, whose district's population is more than 50% 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
President Trump's remarks come just weeks after he was criticised for another tweet telling "'progressive' Democrat congresswomen" to "go back" to the "crime infested" places they came from.
The message was aimed at four lawmakers of colour - Alexandria Ocasio-Cortez, Rashida Tlaib, Ayanna Pressley, and Ilhan Omar - all of whom are US citizens.
What sparked Mr Trump's latest 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.
What further reaction has there been to Mr Trump's tweets?
A prolific critic of Mr Trump, Mr Cummings has 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."
In another tweet, 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".
The response from Republican Party representatives has been muted so far.
But many other Democrats have derided Mr Trump's latest remarks.
Joe Biden, the frontrunner to challenge Trump in 2020's presidential election, scorned the president's tweets as an example of why he is "unfit to hold the office".
In a video shared by The Hill, Elizabeth Warren - another presidential candidate - added that President Trump's "racist" remarks were "insulting both to the congressman and to the people he represents".
More on race in the US
The hashtag #WeAreBaltimore has also been trending on Twitter, with thousands of social media users expressing their solidarity with the city.
Among them was David Simon, a journalist and writer of The Wire, a critically-acclaimed American crime drama series set in Baltimore.
#WEAREBALTIMORE and there's a block party today on my southside street. This is a city of good Americans who deserve more than a grifting, hollow and self-absorbed failure of a man as their president. @realDonaldTrump is a permanent stain on our land. pic.twitter.com/sQAeE7pC1p— David Simon (@AoDespair) July 28, 2019
Since his initial comments, Mr Trump has attacked Mr Cummings in several other tweets.
"Cummings has done nothing but milk Baltimore dry, but the public is getting wise to the bad job that he is doing!," read one.
The president has also retweeted a video shared by actor Terrence Williams which allegedly shows a large mound of rubbish dumped outside a house in West Baltimore.
Mr Trump urged his followers to "take a look" at the "badly run district", signing off the tweet with the hashtag #BlacksForTrump2020.