LONDONDERRY, N.H. — President Trump threatened Friday to invoke the Insurrection Act in American cities and told supporters in New Hampshire they must vote for him to "save democracy from the mob," an escalation of his campaign rhetoric against demonstrators in the streets.

Trump opened his speech in a suburban airport hangar here with a harsh, fiery depiction of major American cities and a detailed monologue about agitators chasing and taunting his supporters and allies outside the White House following his convention speech Thursday night, describing scenes he had viewed on television in vivid detail. Even after he moved on to other topics, he circled back to the cities, sounding apocalyptic at times.

“Look at what happened in New York, look what happened in Chicago. All Democrats. All radical left Democrats,” Trump said. He added: “You know what I say about protesters? Protesters, your ass. I don’t talk about my ass. They’re not protesters, those are anarchists, they’re agitators, they’re rioters, they’re looters.”

Campaign aides said the lengthy remarks about unrest in cities are part of a broader strategy, driven by Trump, in an attempt to win suburban voters and convince Americans that violence in cities is the fault of his Democratic rival, former vice president Joe Biden — and not his. The goal: to convince voters that Trump would like to fix it, and is tougher on criminals but is being blocked by Democratic mayors, and that demonstrators are Biden supporters dangerous to their neighborhoods.

Biden recently condemned violence at protests and has urged calm while expressing support for those taking to the streets in response to the recent police shootings of Black men.

Polls have repeatedly shown a majority of voters do not agree with the president’s handling of race, and some of his most damaging tweets, according to his current and former aides, have focused on race.

But aides say they believe Democratic officials have not taken on violence from protesters forcefully enough — and have shown weak leadership in managing their cities.

Trump aides and allies now regularly share footage of clashes in cities online and reference it in public appearances.

Trump told the crowd in New Hampshire that he had spoken with White House chief of staff Mark Meadows about the Insurrection Act, which permits the federal government to deploy the military domestically. Trump has previously been rebuffed by military and Justice Department advisers who, when he suggested invoking the act, were concerned about politicizing the military and deploying service members in the United States.

The president faces a reelection bid amid double-digit unemployment, a spreading pandemic that has killed about 178,000 Americans, and racial unrest across the country. He has often been accused of spreading division and racial animus with his comments. Most polls show him trailing Biden, but he has narrowed the gap.

“The status quo is not working for him,” said David Axelrod, a political strategist and former top aide to President Barack Obama. “If you look at the metrics, there’s a reason they didn’t talk about the coronavirus or the economy. The basic message they are trying to deliver is that Biden is the enabler. Every bit of disorder is going to be depicted as Democratic disillusion.”

The speech — one night after Trump delivered more serious, by-the-teleprompter remarks in the White House Rose Garden to accept the Republican nomination for president, contained many of his usual flourishes.

Among his proclamations: He resurfaced debunked voter fraud allegations in New Hampshire from 2016, falsely claiming that busloads of people came over from Massachusetts to vote against him. He called for college football to return and said Democrats blaming him for the sport’s demise was a “hoax.” He encouraged the media to call NATO Secretary General Jens Stoltenberg to secure praise of Trump in a meandering riff about his foreign policy positions, adding that he would be pulling more troops out of the Middle East.

He boasted about an endorsement from a New York police officers union and gave an embellished and elaborate account of Sen. Kamala D. Harris, whom he called a “beauty,” losing the Democratic presidential primary. He accused CNN of cutting off its camera when he criticized the network, lampooned the investigation into Russian interference in the 2016 election and mocked Fox News anchor Chris Wallace — drawing supportive boos from the crowd.

The president misleadingly said again that Republicans would always protect preexisting conditions for health care and launched a scorching attack against Biden, painting him as a puppet of the left and for not leaving his basement — and said he “doesn’t even know he’s alive.”

Trump again falsely said “Mexico is paying for the wall” and bragged about crowds lining the runways to pay homage to him. He largely avoided the pandemic during the speech, arguing for the country to reopen and casting his response to the virus as better than Biden would have done.

“They’re all saying, we want Ivanka,” he said, without elaborating who “they” are. “We should have kept the oil in Iraq,” he said at one point, without explaining further.

The president appeared to be in a good mood and regaled the crowd with extensive commentary on how the media commented on his speech the night before.

The speech, not officially a rally but carrying all of the signature songs and signs, was delivered to a crowd packed together in an outdoor airport hangar. Most were not wearing masks, and the crowd booed when an organizer came to the microphone and encouraged the crowd to don them. Guests were not tested for coronavirus before coming into the venue.

Many in the crowd carried signs that said “Peaceful Protester” and “This is a Peaceful Protest,” seemingly mocking the media and crowds for labeling months of protests over police killings of African Americans as peaceful protests. He referenced the signs throughout the night.

Axelrod, like others interviewed for this article, said it was unsurprising to see the president attack crowds of demonstrators, often Black, after years of inflaming racial tensions with incendiary comments. Many voters, he said, are likely to blame Trump for not doing more to calm tensions and improve policing.

“The thing is, Trump is president and he’s been provocative, instead of calming,” Axelrod said. “Voters may think about renewing this guy’s contract.”

But in discussions this week, many of the president’s advisers said they believed the focus on violence in cities would work, particularly after days of violence in Kenosha, Wis.

Meadows told other advisers and allies, some of them said, that he felt better about the president’s political chances after watching Biden take a tougher line against violence in the streets earlier this week in a TV interview — because he argued that public sentiment was forcing Biden to do so.

Political advisers to Trump said they had discussed for days how to best highlight the unrest in campaign ads and that it will be a centerpiece of Trump’s public remarks, ads and the campaign’s messaging in upcoming months.

The advisers spoke on the condition of anonymity to discuss internal campaign deliberations.

Within six hours of agitators chasing and taunting Trump supporters outside the White House Thursday night as they left his speech, campaign advisers were looking for footage to make an ad. Surrogates were told by campaign advisers to highlight the conflict in media appearances.

“VIOLENT RADICALS HAVE TAKEN OVER OUR CITIES,” the ad from the GOP said.

“You can stop them on November 3,” the ad closed, after showing footage of people abusing police officers, heckling and chasing Trump supporters, including Sen. Rand Paul (R-Ky.), with an ominous tone.

Trump’s initial response to protesters in May after the death of George Floyd — first threatening to shoot looters, then days of doing little as cities burned, then clearing peaceful protesters in front of the White House with tear gas before walking to a nearby church for a photo op where he clutched a Bible — was widely criticized, even by some of his supporters.

Trump seemed most animated when talking about the streets outside the White House Thursday night.

“These incredible people from all over the country, all over the world, they walked out to a bunch of thugs — that wasn’t friendly protesters, they were thugs,” he said Friday night. He said he might bring some of the police officers who protected Paul to the White House for a medal ceremony.

Repeatedly, he talked about violence in Portland, Ore.; Minneapolis; New York and other cities, reminding the crowd “these are Democrat-run cities.” He lampooned Muriel E. Bowser, the Democratic mayor of Washington, D.C., as a “disgrace.” He said police in New York were not allowed no to do their jobs and attacked Biden for not talking about law and order at his convention.

“Joe Biden’s supporters are in the street screaming and shouting at bystanders, with unhinged manic rage,” he said, before adding: “Democrats have no complaints about the rioters and the vandals. . . . No one will be safe in Biden’s America.”

Trump said the protesters didn’t even know who George Floyd, the Black man killed by a police officer in Minneapolis, was. Millions have chanted his name on the streets.

Biden, for his part, has spoken more extensively about protesters demonstrating against police violence on Black Americans and has won praise from Floyd’s family.

“The Biden team has to be vigilant,” Axelrod said, adding that Biden should condemn violence against anyone. “He has to take an implacable stand against violence against any group,” he said.