WASHINGTON — Less than a week after President-elect Donald Trump pledged to govern for “all Americans,” Republicans and Democrats spent Monday sparring over Stephen K. Bannon, Trump’s newly appointed chief strategist and senior adviser, who has a history of using a right-wing news platform to spread divisive views.

In appointing Bannon to a top advisory post, the president-elect sparked anger in black, Jewish, and Muslim communities — many of which were already skeptical of the incoming Trump administration. He also created an opening for Democrats and Republicans to raise concerns that Trump is aligning with the “alt-right,’’ which to many critics is a rebranded form of white supremacy.

Bannon, a former Wall Street investment banker who also worked in Hollywood movie production, became executive chairman of Breitbart News in 2012, where he served until joining Trump’s campaign for the White House in August. Under Bannon’s direction, Breitbart has been intensely criticized for its conspiratorial and harshly worded posts, such as “How Muslim Migrants Devastate A Community” and “Does Feminism Make Women Ugly?”

Through a spokesman, Governor Charlie Baker, a Republican who refused to vote for Trump for president, expressed concerns about the selection of Bannon.

“The president-elect has stated that he will focus on unifying the country after a divisive campaign and the governor is concerned that this selection runs counter to that important goal,” said Brendan Moss, a Baker spokesman.

“This appointment is exactly the wrong message for Americans who were fearful that Trump will continue his hate, bigotry, and division of the campaign into the White House,” said US Representative Katherine Clark, a Democrat who represents suburbs north and west of Boston.

Clark has been a target of Breitbart’s ire. Earlier this year, an article on the site said she was “cultural authoritarianism personified” after Clark asked the Justice Department to crack down on Internet harassment.

“That is not what American values are, what we stand for as a country,” Clark said. “If that’s the message the Trump administration took from this campaign — they are wrong.”

Anti-hate watchdog organizations such as the Council on American-Islamic Relations, the Jewish Community Relations Council, the Anti-Defamation League, and the Southern Law Poverty Center have also released statements repudiating Bannon’s appointment.

Trump, who called Bannon a “highly qualified” leader on Sunday, has yet to respond directly to criticism of the future senior adviser. Other members of Trump’s political team, including incoming chief of staff Reince Priebus and former campaign manager Kellyanne Conway, defended Bannon Monday.

“Here’s a guy who’s from Harvard Business School, a 10-year naval officer, from London School of Economics. He’s very, very smart. Very temperate,” Priebus said.

House majority leader Kevin McCarthy, a California Republican, asked Americans not to “prejudge” Bannon. Conway said “people should look at the full resume.”

Bannon, 62, joined the Trump campaign in mid-August, at a time when the candidate’s eventual ascendancy to the White House looked unlikely. He is credited, along with Conway, with helping craft the campaign’s successful strategy of focusing on white voters, especially in rural areas.

However, Bannon’s professional and personal history includes many examples of divisive views on race, gender, and religion.

Breitbart News has a section of articles dedicated to “Black Crime.” After five police officers were shot and killed in Dallas this year, Bannon wrote an article that suggested that African-Americans are “naturally aggressive and violent.” In 2007 court proceedings, he was accused by his former wife under oath in court of not wanting to send their children to a school with Jewish students, an allegation that Bannon denied, according to media reports at the time.

In 2015, weeks after the racially motivated murder of nine black churchgoers in Charleston, Breitbart published an article encouraging people to fly the Confederate Flag “high and proud.”

“While your supporters are trashing the monuments and reputations of the forefathers of so many Americans, Barack, you might just want to remind us again which state of the Union, north or south, your ancestors resided in during the traumatic years 1861- 1865? Or did Kenya not have a dog in that fight?” the article said.

Republican strategist John Weaver, who worked with Trump’s primary rival, Governor John Kasich of Ohio, strongly objected to the Bannon appointment.

“The racist, fascist extreme right is represented footsteps from the Oval Office,” Weaver tweeted Sunday night. “Be very vigilant America.”

Democrats unleashed a flood of criticism. Representative Seth Moulton, who represents the North Shore, denounced Bannon as a “white supremacist” on Monday.

Senator Ed Markey also released a strongly worded statement, and said “there is no place in our society, let alone the White House,” for someone like Bannon.

“If the saying is true and you are the company you keep, Donald Trump has chosen to champion the positions of neo-Nazis, white nationalists and anti-Semites by appointing Steve Bannon as chief strategist and senior counselor,” Markey wrote. “President-elect Trump will forever poison the well with Congress and the American people by appointing a figure who has fueled the rhetoric and activities of hate groups.”

Senator Elizabeth Warren also criticized the appointment, addressing Trump on Twitter: “You said you’d be a ‘President for all.’ Then why are you embracing the ugliness & divisiveness of your campaign?”

Speaking at a press conference Monday afternoon, President Obama said it would be “inappropriate” for him to comment on the incoming president’s White House appointments. But in a general response, Obama said any disagreements that he and Trump have are a “reminder that elections matter, and voting counts.”

Jeremy Burton, the executive director of the Jewish Community Relations Council of Greater Boston, denounced the appointment.

He said Bannon’s selection contradicts Trump’s message, made in his early morning victory speech Nov. 9, in which the president-elect promised to be a leader who helped “bind the wounds of division” in America.

“We’re deeply concerned that the worst rhetoric or hatefulness of the campaign will become the policy of the administration,” Burton said.

He highlighted an early November campaign advertisement run by the Trump team. In the spot, a narrator characterizes Trump as an outsider politician who could usurp the “global special interests” who control the “levers of power in Washington.” As the narrator speaks, images were shown of financial leaders of Jewish faith, billionaire investor George Soros, Goldman Sachs CEO Lloyd Blankfein, and Federal Reserve chair Janet Yellen.