"What does that mean? What does that mean when a child is sitting in their own faeces, can't take a shower? Come on, man. What's that about? None of us would have our children in that position. They are human beings," Cummings said.
That comment appears to be the main reason for Trump's antipathy towards Cummings, which ignited in his extraordinary Twitter rant overnight.
"Elijah Cummings has been a brutal bully, shouting and screaming at the great men and women of Border Patrol about conditions at the southern border, when actually his Baltimore district is FAR WORSE and more dangerous," Mr Trump said.
"As proven last week during a congressional tour, the border is clean, efficient and well run, just very crowded. Cummings' 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 and filthy place.
"Why is so much money sent to the Elijah Cummings district 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!"
That was the context for Blackwell's emotional appearance on CNN.
The host started by reminding his viewers of Trump's recent, similar attacks on minority congresswomen.That was the context for Blackwell's emotional appearance on CNN.
The host started by reminding his viewers of Trump's recent, similar attacks on minority congresswomen.
"Just two weeks ago President Trump attacked four minority congresswomen. 'Why don't they go back to the totally broken and crime-infested places from which they came?' Reminder, three of them were born here; all of them are American," Blackwell said.
"Donald Trump has tweeted more than 43,000 times. He's insulted thousands of people, many different types of people, but when he tweets about infestation, it's about black and brown people."
Blackwell had to pause for 10 seconds to compose himself before continuing, his voice wavering.
Trump's tweets did not come out of nowhere. He published them 15 minutes after Fox News aired a segment critiquing Cummings' outburst at the hearing and showing footage of rubbish on the streets of Baltimore.
"Democrats like Elijah Cummings say they care about how migrants are being treated at the border, but what about the families and people in their own districts?" host Jedediah Bila said, introducing the segment.
"Congressman Cummings was elected to represent west Baltimore, but living conditions at the border are better than most areas in his district; the city lined with abandoned buildings and trash on the streets."
The footage was taken by a Republican strategist, Kimberly Klacik, who told Fox News Cummings' district was the "most dangerous" in the United States and said she saw abandoned homes filled with rubbish that were "attracting rodents".
The President repeated several of her points in his tweets, and later shared the footage with his followers, so it is quite clear he was inspired by the news segment.
Cummings' colleagues in the Democratic Party leapt to his defence and accused Mr Trump of racism.
House Speaker Nancy Pelosi called Cummings a "beloved leader in Baltimore" and a champion of civil rights.
"We all reject racist attacks against him and support his steadfast leadership," she said.
The party's presidential candidates were also quick to respond.
"Representative Cummings is one of the finest people I've ever served with. It is despicable for you to attack him and the people of Baltimore this way. Once again you have proved yourself unfit to hold the office," said the frontrunner, former vice president Joe Biden.
Joe Hildebrand: The fight against Trump is not about racism
One of his rivals, Beto O'Rourke, said Americans were "so much better than this intolerance and hatred".
"In his words and his actions, Trump has been the most openly racist president we've had in modern history," O'Rourke said.
Cummings himself reacted on Twitter. He chose to highlight his work trying to lower prescription drug prices for his constituents.
But Blackwell's response to the President is the one that has resonated.
