PORTLAND, Ore. — A fatal shooting in Portland, Ore., over the weekend led President Trump to unleash a torrent of tweets and attacks on Sunday, capping a volatile week of street violence that is becoming a major theme in the final weeks of the 2020 campaign.On Saturday, a man affiliated with a right-wing group was shot and killed as a large caravan of supporters of Mr. Trump drove through downtown Portland, where nightly protests have unfolded for three consecutive months. No suspect has been publicly identified and the victim’s name has not been released.The shooting came in the same week that a 17-year-old armed with a military-style weapon was charged with homicide in connection with shootings during a protest in Kenosha, Wis., that left two people dead and one injured.The pro-Trump rally in Portland drew hundreds of trucks filled with supporters and adorned with Trump flags into the city. At times, Trump supporters and counterprotesters clashed in the streets, with fistfights occurring and Trump supporters shooting paintball guns from the beds of pickup trucks as protesters threw objects at them.Mr. Trump on Sunday morning posted or reposted a barrage of tweets about the clashes in Portland, with many of them assailing the city’s Democratic mayor, Ted Wheeler. The president retweeted a video showing his supporters shooting paintballs and using pepper spray on crowds in Portland before the fatal shooting. Mr. Trump wrote that “the big backlash going on in Portland cannot be unexpected,” a remarkable instance of a president seeming to support confrontation rather than calming a volatile situation.The shooting immediately reverberated in a presidential campaign now entering its most intense period, and came on the heels of a Republican National Convention in which the president had sought to reframe the 2020 race as a “law and order” election.Over the weekend, officials with Joseph R. Biden Jr.’s campaign were inundated with concern and unsolicited advice from supporters and allies suggesting the need for a forceful and frontal response. Mr. Biden issued a statement on Sunday accusing Mr. Trump of “recklessly encouraging violence,” while condemning “violence unequivocally” himself.“I condemn violence of every kind by anyone, whether on the left or the right,” Mr. Biden said. “And I challenge Donald Trump to do the same.”Mr. Biden will follow up with a speech in Pittsburgh on Monday, and discussions are underway for a possible trip to Kenosha soon. But the Biden campaign wants to avoid being drawn into a prolonged period of focus on unrest in the streets that campaign officials see as an effort by the Trump campaign to distract from the coronavirus pandemic and the resulting economic downturn, which has forced millions into unemployment.At the same time, Chad F. Wolf, the acting secretary of homeland security, on Sunday left open the potential of sending federal law enforcement to quell the unrest in Portland.During an interview on ABC’s “This Week,” Mr. Wolf said “all options continue to be on the table” to deploy more federal agents to Portland despite the strong opposition of local leaders, who say such tactical teams have only heightened tensions.Mr. Wheeler, at an afternoon news conference at City Hall, said the shooting had left his heart heavy, and he denounced violence. But he pointed to Mr. Trump’s combative and unyielding message as a generator of the nation’s escalating polarization and violence, and he called on the president to work with him and others to help de-escalate tensions.“Do you seriously wonder, Mr. President,” he said, “why this is the first time in decades that America has seen this level of violence? It’s you who have created the hate and the division.”He added: “We need to reset. The president needs to reset. I need to reset. This community needs to reset. And America needs to reset. And it’s going to take his leadership in the White House and it’s going to take my leadership here in City Hall to get it done.”Mr. Trump responded quickly to the mayor’s remarks, mocking Mr. Wheeler and calling him “wacky” and a “dummy.”“He would like to blame me and the Federal Government for going in, but he hasn’t seen anything yet,” the president wrote on Twitter.Mr. Trump is planning to visit Kenosha on Tuesday, though both the governor of Wisconsin, Tony Evers, a Democrat, and the mayor of Kenosha, John Antaramian, also a Democrat, urged him to reconsider. “I am concerned your presence will only hinder our healing,” Mr. Evers said.Judd Deere, a White House spokesman, said: “The White House has been humbled by the outreach of individuals from Kenosha who have welcomed the president’s visit and are longing for leadership to support local law enforcement and businesses that have been vandalized. President Trump looks forward to visiting on Tuesday and helping this great city heal and rebuild.” The shooting in Portland ended a week of upheaval that began when a white police officer in Kenosha repeatedly shot a Black man, Jacob Blake, leaving him paralyzed below the waist, his family said. The shooting prompted a new wave of protests against racism and police brutality that included the postponement of professional sports games.During the unrest after the shooting of Mr. Blake, Kyle Rittenhouse, a 17-year-old Illinois resident, was charged in connection with the fatal shootings of two protesters.The escalating tensions and violence over the past week came three months after the killing of George Floyd in police custody in Minneapolis. Mr. Floyd’s death prompted a national outcry over policing and racial injustice, spurring protests in cities across the country, some of which have been accompanied by looting and violence.For now, the Biden campaign is trying to focus on what it says is the irony that Mr. Trump is the current president, yet is trying to blame his challenger for the scenes of violence during his tenure.“He keeps talking about what Biden’s America would look like — well, this is Trump’s America,” Representative Cedric L. Richmond, Democrat of Louisiana and a national co-chair of the Biden campaign, said on NBC’s “Meet the Press.”But violence and unrest in the streets is an issue Mr. Trump is eager to embrace.On “Meet the Press,” Mark Meadows, the White House chief of staff, described Mr. Trump as being “on the side of law enforcement and the rule of law” and spoke of violence in “Democrat cities.”“Most of Donald Trump’s America is peaceful,” Mr. Meadows said. “It is a Democrat-led city in Portland that we’re talking about this morning who just yesterday denied help once again from the federal government.”A video that purports to capture the Saturday night shooting in Portland, taken from the far side of the street, showed a small group of people in the road outside what appears to be a parking garage. Gunfire erupts, and a man collapses in the street.The man who was shot and killed was wearing a hat with the insignia of Patriot Prayer, a far-right group based in the Portland area that has clashed with protesters in the past. Joey Gibson, the head of the group, said Sunday he could not share many details but could confirm the man was a good friend and supporter of Patriot Prayer.During some of Portland’s nightly demonstrations since the killing of Mr. Floyd, protesters have smashed windows, lit fires and thrown fireworks at law enforcement officers who have struggled to maintain control. In recent days, right-wing demonstrations have also sprung up in the city, and Mr. Trump has repeatedly highlighted the unrest in Portland as evidence of the need for a tougher response to the chaotic protests in many American cities.Patriot Prayer, which says it promotes Christianity and smaller government, has repeatedly clashed with activists in Portland. The group has sometimes operated alongside militia groups, and the Southern Poverty Law Center has reported that some Patriot Prayer events have drawn white supremacists. Last year, Mr. Gibson, the group’s leader, was charged along with others with rioting after a brawl in the city.While protests in Portland have persisted, their size has changed over time. The nightly events began with mass demonstrations after Mr. Floyd’s death, then shrank to smaller numbers of people who repeatedly clashed with the police. In July, when the federal government sent camouflaged agents into the city, the number of protesters grew drastically once again.In more recent days, the protest crowd has typically numbered just a few hundred people. On Friday, after a peaceful demonstration in front of Mayor Wheeler’s residence, a crowd went to a police association building, where some of the protesters set fire to the front of the building before the police dispersed the crowd.The police have made dozens of arrests in recent days as they have chased protesters through the streets, at times knocking them to the ground. The police said they had made 10 arrests Saturday night, although it was not immediately clear how many were participants in the pro-Trump rally and how many were countering the event.Mike Baker reported from Portland, Thomas Kaplan from Connecticut and Shane Goldmacher from New York. John Eligon contributed reporting from Kenosha, Wis., and Katie Glueck from New York.