U.S. President Donald Trump, again stoking racial divisions, renewed his attacks Monday on a black U.S. congressman from Baltimore and slammed black civil rights activist Rev. Al Sharpton, drawing a rebuke from Maryland's Republican governor and the black former Republican Party national chair.

In a series of Twitter posts, the Republican president directly tied his criticism of Rep. Elijah Cummings and Sharpton, as well as his ongoing attacks on four racial and ethnic minority Democratic congresswomen dubbed the "squad," to his 2020 re-election bid.

Democrats have called Trump's weekend remarks disparaging Cummings, a House of Representatives Democrat and a longtime civil rights champion, and his comments about the congresswomen racist — an accusation Trump denied. On Twitter, he has called the majority-black congressional district in Baltimore that Cummings represents "a disgusting, rat and rodent infested mess."

Maryland Gov. Larry Hogan, a Republican who had considered challenging Trump for the party's 2020 presidential nomination, blasted Trump's tweets in an interview with WBAL radio in Baltimore and said the president's latest Twitter attacks were only further fuelling division and political anger nationwide.

"The comments are just outrageous and inappropriate," Hogan said. "I think enough is enough."

'Water off a duck's back'

On Monday, Trump again slammed Cummings and his district over its crime rate.

Many Republicans have refrained from criticizing Trump over his remarks toward Cummings. But Michael Steele, a former lieutenant governor of Maryland who served as chair of the Republican national committee from 2009 to 2011, took aim at Trump at a news conference in Baltimore alongside Sharpton.

"Mr. President, your reprehensible comments are like water off a duck's back when it comes to this community. It just washes off of us," said Steele, who is black.

Put the tweet down brother, and show up. - Michael Steele, former lieutenant governor of Maryland

Steele urged Trump to visit Baltimore, saying, "Folks want to talk to you. So just show up. Put the tweet down brother, and show up."

Cummings, chair of the House oversight and reform committee, is leading a number of congressional investigations into Trump and his administration, and has criticized the president's handling of migrants.

He has also drawn the president's ire for investigations touching on his family members serving in the White House. His committee voted along party lines Thursday to authorize subpoenas for personal emails and texts used for official business by top White House aides, including Ivanka Trump and her husband, Jared Kushner.

The president lashed out at Rep. Elijah Cummings on Twitter over the weekend, calling the majority-black congressional district in Baltimore that he represents 'a disgusting, rat and rodent infested mess.' (J. Scott Applewhite/The Associated Press)

Baltimore Mayor Bernard Young said Trump should take action to help U.S. cities.

"Stop tweeting and send federal help and federal resources to the city of Baltimore. Not just to the city of Baltimore, to urban cities around the country that are having problems with aging infrastructure, with crime and grime, and reduction in resources from the federal government," Young told MSNBC.

Trump also targeted Sharpton, who has defended Cummings and the congressman's district. Sharpton and Trump have ties going back to their New York City circles, but Trump balked at any close past relationship.

"Just a conman at work!" Trump wrote on Twitter.

U.S. President Donald Trump has been called racist for his Twitter tirade against Elijah Cummings, Rev. Al Sharpton and several racial and ethnic minority Democratic congresswomen. He denies that accusation. (Leah Millis/Reuters)

At the news conference, Sharpton condemned Trump for what he called "a bigoted and racist" attack on Cummings and the people of Baltimore.

"He can say what he wants. Call me a troublemaker, yes: I make trouble for bigots," Sharpton said.

Meanwhile, Trump tweeted that he was convening a group of "wonderful Inner City Pastors" for an unannounced closed-door meeting to discuss the issues facing the black community Monday.

"This country needs healing. There's so much division in America along racial lines," said Bill Owens, president of the Coalition of African-American Pastors, who said he was among about 20 pastors who met with the president.

"He wanted to know from us: What should he do in America? What best can he do?" Owens said of Trump, insisting the gathering "was not damage control."

Asked by a reporter whether he thought Trump was racist, Owens said he found that "hard to believe," but said the president could do more to address racism in the country, "absolutely."

Trump made clear his tweets targeting Cummings and "squad" members Alexandria Ocasio-Cortez of New York, Ilhan Omar of Minnesota, Ayanna Pressley of Massachusetts and Rashida Tlaib of Michigan were tied to the November 2020 presidential election.

"If the Democrats are going to defend the Radical Left 'Squad' and King Elijah's Baltimore Fail, it will be a long road to 2020," Trump tweeted on Monday.

Like in his 2016 campaign, Trump has put race front and centre while Democrats also grapple with the issue as they seek to select a 2020 presidential nominee from nearly two dozen candidates, including several racial minorities and women.

Trump's acting chief of staff Mick Mulvaney on Sunday defended the president, saying the tweets against Cummings were not racist.