WASHINGTON — Former Vice President Joseph R. Biden Jr. has called President Trump an “existential threat” to the nation.
Senator Kamala Harris has warned on the campaign trail that the president is a racist.
After the mass shooting in El Paso this month, in which a Texas man who confessed to killing 22 people echoed anti-immigrant language promulgated by Mr. Trump and right-wing media outlets, Senators Elizabeth Warren and Bernie Sanders, as well as former Representative Beto O’Rourke, escalated their verbal attacks on Mr. Trump, labeling him a “white supremacist.”
Each contender for the 2020 Democratic presidential nomination is trying to distinguish himself or herself as the strongest candidate to take on the president. In doing so, the candidates have been eager to be seen as speaking plain truth to what they frame as venal power on issues of race, character and everything else.
“Donald Trump believes climate change is a hoax,” Mr. Sanders wrote on Twitter over the weekend, while campaigning at the Iowa State Fair. “Donald Trump is an idiot.”
But if the steady stream of ad hominem attacks is failing to shock, awe or drive the news, it is in part because there is little left in terms of criticism of Mr. Trump that has not already been volleyed — by his own team.
In 2015, Senator Lindsey Graham, Republican of South Carolina, labeled Mr. Trump a “race-baiting, xenophobic, religious bigot” and called him the “ISIL man of the year,” referring to the Islamic State. That was in addition to describing him as “kook,” a “crazy” and a man who was “unfit for office.”
Senator Ted Cruz, the second-to-last man left standing in the ugly 2016 Republican primary race, called Mr. Trump a “pathological liar” who was “utterly amoral,” a “serial philanderer” and a “narcissist at a level I don’t think this country’s ever seen.”
Mick Mulvaney, the former Republican congressman who now serves as the president’s acting chief of staff, in 2016 called him a “terrible human being” who had made “disgusting and indefensible” comments about women.
Revisiting the now hollow critiques from Mr. Trump’s own party — those Republicans have all embraced the president and his agenda, going on to serve in his administration or offer him support from Capitol Hill — is raising questions about whether the search for a killer line on Mr. Trump is a fool’s errand.
“Voters on the whole are completely desensitized to personal attacks on Trump at this point,” said Tim Alberta, the author of “American Carnage: On the Front Lines of the Republican Civil War and the Rise of President Trump.”
“Rick Perry called him a ‘cancer’ and then became a cabinet secretary,” he said. “It’s not like a swing voter in a battleground state will hear an ad hominem attack on him and suddenly think, ‘I never thought of it that way.’”
Mr. Alberta, who in researching his book spent time Monday-morning quarterbacking the 2016 election with many of the failed Republican candidates, said that “the unlearned lesson from 2016 for Republicans was that every day spent launching ad hominem attacks on Trump was a day not spent pointing to voters how little he knows about actually running the government.”
While the attacks may help Democrats distinguish themselves from one another, they appear to have no effect on Mr. Trump’s poll numbers, according to officials. Mr. Trump’s internal approval ratings, a source familiar with the numbers said, have not shifted by more than 1 percent since April 2018, and hover between 43 and 45 percent.
“Everything is tribal at this point,” said Brendan Buck, a former adviser to Paul Ryan, the former House speaker. “If you’re with him, you’re with him, in spite of or because of the way that he is.” But Mr. Buck said that since 2016, there has been “slippage with women voters, suburban voters — and I think some of it is related to his persona and the way he presents himself” and the character that Democrats are critiquing.
Trump campaign officials shrug off the race to be seen as the biggest Trump critic in the Democratic Party. They view it as an intraparty fight between candidates trying to raise small dollars and qualify for the next presidential debate, but they have tracked no negative effect on Mr. Trump’s standing because of it.
And they have ignored the president’s one Republican opponent, William F. Weld, the former Massachusetts governor waging the longest of long-shot Republican primary challenges, who also appears to be recycling the attacks that failed to stop Mr. Trump’s capture of the party’s nomination in 2016. “Donald Trump is a raging racist,” Mr. Weld said last month. “He’s a complete and thoroughgoing racist.”
Mr. Trump, for his part, has bristled at being labeled a racist, but has not indicated to his allies or advisers that he intends to change his language or reach out to communities of color with a message of unity.
Democratic strategists, however, said there was a difference between a voter’s expectations of a sitting president and a first-time candidate.
“Part of the reasons that voters in 2016 overlooked a lot of these same criticisms of Trump is that they hoped that his schtick was just a schtick,” said Geoff Garin, a Democratic pollster. “It’s possible these criticisms will be more meaningful because it’s much harder to have the illusion that these character deficiencies aren’t for real.”
Mr. Garin, who worked for Ralph Northam in 2017 during Mr. Northam’s Democratic primary race for governor of Virginia, said attacking Mr. Trump also helps centrist candidates prove themselves to progressive voters.
Mr. Northam’s primary race opponent, former Representative Tom Perriello, challenged him from the left, Mr. Garin said, “but Ralph said in a commercial that Donald Trump is a narcissistic maniac.”
“That’s all that Democratic primary voters needed to hear to know he was in tune with them,” he added.
Among the Democratic candidates for president, there is an awareness that Mr. Trump will always win in a game of diminishing nicknames and threatening speech. But many of them view it as a moral obligation to attach a label to him.
“At a time when the president’s words are being weaponized by violent extremists, it’s important to speak in direct terms about who he is,” said Sawyer Hackett, the national press secretary for Julián Castro, a presidential candidate and former housing secretary. “He is a racist who fans the flames of white nationalism and white supremacy.”