President Donald Trump pushed back Sunday against Democrats after they denounced as racist his attacks on Baltimore as a "rodent infested mess" where "no human being would want to live."

Trump said his opponents "always play the race card when they are unable to win with facts."

It was the second time Sunday that Trump accused the Democrats of playing the "race card." But in his next tweet he called Rep. Elijah Cummings, D-Md., a "racist" who should "focus more of his energy on helping the good people of his district, and Baltimore itself." He did not explain why he thought Cummings was a racist.

Trump made his original comments about Baltimore on Saturday in a series of tweets criticizing Cummings because the chairman of the House Oversight Committee decried the administration's treatment of migrants at the U.S.-Mexican border.

Trump slammed Cummings' district, which includes much of Baltimore, as a "filthy place" that is "FAR WORSE and more dangerous" than the southern border. He called it the worst district "in the USA."

Democrats rallied to defend Cummings and denounced the president's tweets attacking an African American congressman and a city that is more than 60% African American as racist.

House Speaker Nancy Pelosi, D-Calif., who was born in Baltimore, said Cummings "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."

"There is nothing racist in stating plainly what most people already know, that Elijah Cummings has done a terrible job for the people of his district, and of Baltimore itself," Trump tweeted. "Dems always play the race card when they are unable to win with facts. Shame!"

Despite his protestations against the use of the label, Trump frequently accuses his critics of racism.

This month, after Trump posted tweets telling four minority congresswomen to "go back and help fix" their countries of origin, Pelosi said the remarks showed Trump's "plan to 'Make America Great Again' has always been about making America white again." The president called that a "very racist statement."

He also called the four Democrat congresswomen "a very Racist group of troublemakers who are young, inexperienced, and not very smart."

Others he has accused of racism include former President Barack Obama, Sen. Elizabeth Warren, D-Mass., director Spike Lee, commentator Tavis Smiley, sportscaster Bryant Gumbel, former Rep. Anthony Weiner, D-N.Y., comedian Jon Stewart, "Modern Family" producer Danny Zucker, the film "Django Unchained" and Macy's department store.

Then came the tweet Sunday about Cummings.

Fox News' Chris Wallace:'No question' Trump is 'stoking racial divisions'

More:Trump calls congresswomen racist day after his adviser says the term is used to silence speech

Sen. Chris Murphy, D-Conn., declared Sunday that he is unfollowing President Donald Trump on Twitter because the president's feed "is the most hate-filled, racist, and demeaning" that he sees on the social media site.

"It regularly ruins my day to read it. So I'm just going to stop," he said in a tweet.

"I can't believe I just typed that," he added.

Murphy was not the only politician expressing anger at Trump's tweets this weekend.

'They are human beings':Cummings to acting DHS chief over migrant detention facilities

"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," Cummings said in a tweet on Saturday.

Baltimore Sun responds to Trump insults:'Better to have a few rats than to be one'

More:Pelosi denounces Trump's 'racist attacks' on Rep. Elijah Cummings and Baltimore

When asked on ABC News' "This Week" whether a House resolution might be introduced in response to Trump's tweets about Cummings and his district, Rep. Jerry Nadler, D-N.Y., said, "It wouldn't be a bad idea."

Presidential candidate Sen. Bernie Sanders, I-Vt., said Sunday on CNN's "State of the Union" that "it's unbelievable that we have a president of the United States who attacks American cities."

"We have rural Republican districts where life expectancy is going down, where downtowns are boarded up, where people are struggling," Sanders said.

He called Trump "a racist president who attacks people because they are African Americans. That is a disgrace. And that is why we're going to defeat this president."

Trying to highlight economic inequality on a tour of the city during his presidential campaign in 2015, Sanders compared an area in Baltimore to a "third-world country," according to The Baltimore Sun.

Cummings:Trump doesn't know how it feels to be treated 'like less than a dog'

Who is Rep. Elijah Cummings?:What you need to know about the chairman of the House Oversight Committee

On CBS News' "Face the Nation," acting White House Chief of Staff Mick Mulvaney said Trump posted his tweets to "push back against what the president sees as inaccuracies" in things Cummings said about the conditions at the border.

Mulvaney echoed the president's sentiment that Cummings should focus on the problems in his district instead of spending time on the Oversight Committee's investigations into the administration.

"Does the president speak hyperbolically? Absolutely," Mulvaney said. "Have we seen this type of reaction for him before? Yes, and you will again because he pushes back. He fights back when he feels like he's attacked, and what Mr. Cummings said this week was wrong."

When asked whether he understood why people were offended by the tweets, Mulvaney said, "I understand that everything that Donald Trump says is offensive to some people."

Rep. Will Hurd of Texas – one of four Republicans to vote with Democrats in condemning Trump's tweets against the four minority congresswomen as racist – said Sunday that the tweets about Baltimore did not rise to the same level.

Hurd said he "wouldn't be tweeting the same way," and Cummings is "someone that cares passionately about his community," but "I think these tweets are different."

He said he didn't expect the Baltimore Orioles would ask Trump "to throw out the first pitch at a baseball game anytime soon."

Democratic presidential hopeful Julián Castro said on "Face the Nation" that he is not quick to label someone a racist, and "you have to be very careful before you use that word," but he said there is "a pattern" of behavior with Trump.

"This guy is the biggest identity politician that we have seen in the last 50 years, and he engages in what's known as racial priming – basically using this language and taking actions to try and get people to move into their camps by racial and ethnic identity," said Castro, a former secretary of the Department of Housing and Urban Development. "That's how he thinks he won in 2016, and that's how he thinks he's going to win in 2020."

Rep. Rashida Tlaib, D-Mich., was born in Detroit, where her parents, who are Palestinian immigrants, settled. She was one of the four lawmakers Trump told to fix the "crime infested places from which they came" before "telling the people of the United States, the greatest and most powerful Nation on earth, how our government is to be run."

On "State of the Union," Tlaib said, "Our president has a hate agenda. He doesn't have a policy agenda." She said that "hate agenda is now seeping into policymaking," accusing the president of "making decisions based on profits" for "his friends" and the Trump Organization "versus what is best for the American people."

New York City Mayor Bill de Blasio, another candidate seeking the Democratic presidential nomination, said Trump's tweets were a racist "bait and switch" from a man with a "con-man reality" intended to distract from the "real issue in this country," which he said is "that working people are stuck economically."