Mr Trump unleashed a flurry of tweets and retweets the day after a man identified as a supporter of a right-wing group was shot and killed in Portland.
The city has been the site of daily protests for months, and a large caravan of Mr Trump’s supporters and Black Lives Matter protesters clashed on Saturday night.
The president praised the caravan participants as “GREAT PATRIOTS!” and retweeted what appeared to be the dead man’s name along with a message to “Rest in peace”, as well as retweeting those who blamed the city’s Democratic mayor for the death.
Mr Trump wrote: “The people of Portland, like all other cities & parts of our great Country, want Law & Order.
“The Radical Left Democrat Mayors, like the dummy running Portland, or the guy right now in his basement unwilling to lead or even speak out against crime, will never be able to do it!”
He may think that war in our streets is good for his reelection chances, but that is not presidential leadership — or even basic human compassion
Mr Trump has sought to cast cities as under siege by violence and lawlessness, despite the fact that most of the demonstrations against racial injustice have been largely peaceful.
With about nine weeks until Election Day, some of his advisers see an aggressive law and order message as the best way for the president to turn voters against his Democratic rival, Joe Biden, and regain the support of suburban voters.
But Democrats accused Mr Trump of rooting for unrest and trying to stoke further violence for political gain instead of seeking to ratchet down tensions.
“He may think that war in our streets is good for his reelection chances, but that is not presidential leadership — or even basic human compassion,” Mr Biden said.
Portland Mayor Ted Wheeler, a Democrat, blamed Mr Trump for the tensions.
“Do you seriously wonder, Mr President, why this is the first time in decades that America has seen this level of violence?” he asked at a televised news conference.
“It’s you who have created the hate and the division.”
The circumstances of Saturday night’s shooting in Portland remain unclear.
Video from the city shows sporadic fighting between groups, with Mr Trump’s supporters firing paintball pellets at opponents and using pepper spray as counter-protesters threw things at the Trump caravan.
The man killed was a member of Patriot Prayer, a right-wing group whose members have frequently clashed with protesters in Portland in the past, its founder, Joey Gibson, said on Sunday.
He identified the victim as Aaron ‘Jay’ Danielson and called him a “good friend,” but provided no details. Mr Danielson apparently also went by the name Jay Bishop, according to Patriot Prayer’s Facebook page.
Mr Trump retweeted the victim’s name and wrote: “Rest in peace Jay!”
