Over the course of 72 hours, President Trump’s campaign accused Joseph R. Biden Jr. of plagiarism. Mr. Trump warned that a Biden presidency would lead to a surge of crime in the streets. He tried to link his opponent to socialism and to Representative Alexandria Ocasio-Cortez, a leader of the Democratic left. And he said that Mr. Biden would eliminate the suburbs — and windows.
Facing weak poll numbers and criticism for failing to offer a second-term agenda or a cohesive case against Mr. Biden, the president is accelerating his attacks on his Democratic opponent — a sign of nervousness for any incumbent — as he looks for a way to turn the corner for his struggling candidacy. He has shaken up his campaign staff and intensified a tear-down operation aimed at Mr. Biden with a dizzying barrage of attacks, highlighted by dark, and at times misleading, television ads.
Deprived of his favored forum of raucous campaign rallies because of the coronavirus pandemic, Mr. Trump has road-tested his messages in the Rose Garden, at a staid appearance in Atlanta to announce rollbacks of environmental regulations and on Twitter, supplying an onslaught of scattershot and sometimes contradictory criticisms of the former vice president.
“It’s definitely a step in the right direction,” said Neil Newhouse, a Republican pollster. He noted the contest had focused on the president’s record and conduct for months, and that the Trump campaign needed to begin “reminding people that this is a choice election, and beginning to frame up what that decision entails.”
Mr. Trump and his political team have tried new lines of attack before, including mocking Mr. Biden’s mental faculties, portraying him as corrupt and arguing he is too cozy with China — none of which reversed Mr. Trump’s slide in the polls.
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The president’s lack of discipline is a caution against any Republican hopes that this might be the start of a new chapter. The latest attacks on Mr. Biden may not work with undecided voters who have relatively positive views about him and his moderate political message, and are unhappy with Mr. Trump’s performance. The president’s record of falsehoods, some strategists said, undercut his credibility with the American public over the past four years, and thus his ability to deliver effective criticism of Mr. Biden.
Aides to Mr. Trump said he has become increasingly aware of his peril and has leaned even more into the day-to-day operations of his campaign. The latest evidence of that was his decision on Wednesday night to remove Brad Parscale as his campaign manager and to replace him with Bill Stepien, the deputy. Mr. Stepien is close to Jared Kushner, the president’s son-in-law and senior adviser, who has been put firmly at the helm of the re-election effort.
The new stance could represent a calculated effort by a campaign looking for a reset before it is too late. There is a long history of presidents using this period before Labor Day to set the framework of the race, and to negatively define their opponents; Mr. Trump has been under criticism from Republicans for his failure to do that.
But there’s skepticism among Republicans and Democrats that Mr. Trump can reverse his troubled political trajectory. Beyond the overwhelming problems from the pandemic, the president lacks any kind of strategy to win over voters who are not already part of his right-wing base — or even, apparently, the will to do so — and his polarizing style has left him viewed unfavorably by much of the country, including in some traditionally Republican strongholds.
The broad and random nature of his attacks against Mr. Biden over the past few days suggest that Mr. Trump, unlike George H.W. Bush in 1988 and George W. Bush in 2004, has not engaged in the kind of methodical research that can identify the best lines of attack against an opponent. In Philadelphia this week, Mr. Trump ran television advertisements that accused Mr. Biden of both being soft on crime and being too tough on crime.
“If you go back to President Bush and President Obama, they effectively already had a bead on their opponent, a cleareyed strategy of what they were going to attack on, how they were going to frame it and how to make the election a binary choice,” said Rahm Emanuel, a Democrat who served as chief of staff to President Barack Obama and as mayor of Chicago. “That was a year and a half out. This is four months out. Trump is still trying to figure out how to get his handle on Joe Biden.”
Mr. Trump’s impulsive style of campaigning must be taken into account as well, strategists from both parties said.
“We shouldn’t overthink any of this as a strategy,” said Kevin Madden, who was a senior adviser to Mitt Romney, the Republican candidate for president in 2012. “It’s all the reflexive action of a candidate reacting to the hourly news cycle and trying to program it.
Mr. Trump’s newly intensified efforts to define Mr. Biden pose a strategic challenge to the Biden campaign in determining how — or if — to respond.
Mr. Biden’s aides and allies argued that Mr. Trump had so many vulnerabilities that he was limited in what kind of attacks he could make. They said they would be selective in deciding when to respond, looking for ways to advance their own message while trying to avoid elevating false claims.
“Donald Trump has spent his entire life depending on wild-eyed lies and asinine conspiracy theories to distract from his own failures and wrongdoing,” Andrew Bates, a spokesman for Mr. Biden, said. “We starve Trump of the engagement that he wants and then pounce when he shows his underbelly.”
From this perspective, Mr. Trump has become so discredited with much of the American public that many of the attacks will come across as desperate.
“If you don’t know what sticks by now, it’s too late,” said Stephanie Cutter, a Democratic consultant who is helping to oversee the nominating convention for Mr. Biden. “When your opponent is throwing spaghetti to the wall to see what sticks, you don’t respond, you stick to your own strategy.”
“These attacks are not convincing for anybody on the fence,” she said. “For the Democratic base, and anybody on the fence who may have voted for Trump, these attacks are not what the race is about. It’s not about socialism, it’s not about defunding the police, it’s not about Bernie Sanders. It’s about moving this country away from the chaos and corruption of the last four years.”
But Democrats and Mr. Biden’s aides are well aware of the history of presidential candidates who ignored attacks they thought were specious and then lost the election — like John F. Kerry, in 2004, and Michael Dukakis, in 1988, both Democrats.
A claim that Mr. Biden supports efforts like those to defund the police, some Democrats said, could prove to be potent with at least a slice of voters and was worth watching, even though Mr. Biden has said repeatedly that he opposes defunding the police.
In recent weeks, two of Mr. Biden’s top advisers — Steve Ricchetti and Representative Cedric Richmond of Louisiana — have set up Zoom meetings with Democratic congressional delegations in key battleground states. It’s an effort to open conversations with key leaders on the ground who can also serve as an early warning system if Mr. Trumps’ attacks are getting traction, some participants said.
If necessary, Mr. Biden could move to rebut some of Mr. Trump’s attacks; he could also try to change the subject by mounting his new attacks on Mr. Trump, reflecting the adage in politics that a candidate who is defending is a candidate who is losing.
“They’re setting up very strong lines of communication,” said Representative Debbie Dingell, a Michigan Democrat. “If somebody does get worried, they’re hearing it, paying attention. They were really clear about that: They want to have those lines of communication open.”
Ms. Dingell said she has warned the Biden campaign that the Trump campaign’s most searing attacks cannot go unanswered.
“He is clearly playing ads that are wedge issues, and we have to be prepared to answer them, which we didn’t four years ago,” she said, pointing to trade, an issue of particular importance to autoworkers in her state. “We’re doing better than I thought we might be at this point, but the election is still three months away and we have to be prepared.”
Mr. Biden’s team has been through clashes with Mr. Trump before — such as last fall, when the president laced into the former vice president’s son, Hunter, criticizing his work for a Ukrainian energy company. At the time, the campaign struggled to form a forceful response, paralyzed by indecision.
Since then, advisers and allies say, they have honed a playbook of sorts, reflecting a growing understanding of how to be nimble and challenge apresident who is capable of drawing significant attention to his slashing attacks.
The best approach, said Representative Tim Ryan, Democrat of Ohio, is, “kind of letting him flail around, but there’s always, within those tracks, opportunities to hit back. And I think they’ve been doing it.”