As the campaign takes shape about 15 months before voters render a verdict on his presidency, Mr Trump's Democratic challengers are marking him a racist, and a few have gone so far as to designate the president a white supremacist.
Throughout his career as a real estate magnate, a celebrity provocateur and a politician, Mr Trump has recoiled from being called the r-word, even though some of his actions and words have been plainly racist.
Following a month in which he levelled racist attacks on four congresswomen of colour, maligned majority-black Baltimore as a "rat and rodent infested mess" and saw his anti-immigrant rhetoric parroted in an alleged mass shooter's statement, the risk for Mr Trump is that the pejorative that has long dogged him becomes defining.
Being called a racist has infuriated Mr Trump, gnawing at him in recent days as he lashes out - in tweets and in public comments - over the moniker, behaviour his advisers and allies excuse as the natural reaction of anyone who does not consider himself a racist but is accused of being one.
"For them to throw out the race word again - racist, racist, racist," Mr Trump told reporters Friday as he departed the White House for a week-long vacation at his private golf club in Bedminster, New Jersey. "They call anybody a racist when they run out of cards."
The president views the characterisation largely through the lens of politics, said one close adviser, explaining that Trump feels the charges of racism are just another attempt to discredit him - not unlike, he believes, the more than a dozen women who have accused him of sexual misconduct or the Russia investigation.
Many of his supporters see it the same way. "At first, they tried to use Russia, and that didn't work," said Don Byrd of Newton, Iowa. "Now it's all about race - 'He's a racist. He's this. He's that.' "
Democrats have engaged in semantic manoeuverings about just how racist they think the president is. While former congressman Beto O'Rourke and Sen. Elizabeth Warren of Massachusetts said without hesitation that the president is a white supremacist, former Vice President Joe Biden stopped short.
"Why are you so hooked on that?" Mr Biden told reporters last week in Iowa. "You just want me to say the words so I sound like everybody else. I'm not everybody else. I'm Joe Biden. ... He is encouraging white supremacists. You can determine what that means."
Mr Trump's allies argue Democrats risk overreach in maligning the president.
"Democrats seem to forget that Trump supporters include blacks, whites, Hispanics and other minority groups who simply love this country," said Mercedes Schlapp, a Trump campaign adviser, in a text message. "Democrats have shown their absolute disdain for the president and now they have extended their disdain to half of America."
Some Democrats seem cognisant of the danger. At last month's presidential debate, Senator Amy Klobuchar of Minnesota said, "There are people that voted for Donald Trump before that aren't racist; they just wanted a better shake in the economy."
Yet, she, too, also felt the need to rebuke Trump. "I don't think anyone can justify what this president is doing," Ms Klobuchar concluded.
Mr Trump recently called himself "the least racist person anywhere in the world," but his history is littered with racist and racially charged comments and actions.
In 1989, Mr Trump purchased newspaper advertisements demanding the death penalty for the "Central Park Five", black and Latino teenagers wrongly accused of raping a jogger in New York.
In 2005, he pitched a culturally divisive spin-off of his popular reality television series: "The Apprentice: White People vs. Black People."
Trump then rose to political prominence partially by championing the racist birtherism lie that former president Barack Obama was born outside of the United States. As a presidential candidate, Mr Trump attacked a judge overseeing a Trump University case for his Mexican heritage.
Last month, Mr Trump tweeted that four minority congresswomen known as "the Squad" should "go back" to the "totally broken and crime infested places from which they came," even though three of the four lawmakers were born in the United States.
Mr Trump's rhetoric came under fresh examination last week after the alleged gunman who killed at least 22 people in El Paso, Texas, echoed, in what is believed to be his missive, Mr Trump's language about an "invasion" of Hispanic migrants.
People who know Mr Trump have come to his defence. Kellyanne Conway, counsellor to the president, said that, in her three years at his side, she has "never, ever, a single time heard this president say or do anything" racist.
She described his reaction to being labelled a racist as "less frustration and more consternation that critics, especially those who would like to be president, resort to spewing invectives or hurling insults at the current president, instead of just arguing on the issues".
Trump's sensitivity about the racist sobriquet dates back decades. Al Sharpton, a civil rights activist who has known Trump and tangled with him for many years, said the president has long understood that being called "the r-word" would damage his casino and hotel businesses and, now, his political standing.
"At one level, you're super sensitive about the r-word, and on another level, you buy ads on the Central Park Five," he said.
Mr Sharpton recalled that, at the height of the birtherism debate, Mr Trump sought to persuade him to stop calling him out for his lies about Mr Obama's birthplace on his MSNBC show by inviting him to a meeting at Trump Tower.
"I'm not a racist," he recalled Mr Trump adamantly insisting.
The two men argued and Mr Sharpton responded, "I'm not calling you a racist, but what you are doing is racist."
Mr Sharpton continued to attack Trump on air.
Some people who have worked for Trump say the president is less concerned about the moral significance of being called a racist but focuses instead on the bottom-line implications.
"The guy sends out blatantly racist tweets," former White House communications director Anthony Scaramucci said. "White supremacist. Racist. Those labels are bad for business. ... It means a reduction in the colours of people who want to vote for you. He's upset about it because it's bad for business."
To the extent that one's understanding of what is and isn't racist is forged in his youth, Trump's upbringing may be instructive. One former adviser suggested Trump believes he is more racially tolerant than his father, Fred Trump, who was reported to have been arrested in connection with a 1927 Ku Klux Klan march in New York - an arrest the president has denied as "nonsense" and "never happened".
In the 1970s, Fred and Donald Trump were both sued by the Justice Department for discriminating against black renters in their residential properties.
Ms Conway argued the charges of racism against Trump are over the top and that they are likely to help him politically because his voters could think Democratic candidates are unfairly branding them as racists, too, simply for supporting the president.
"When the elite wrist-flickers are out there demeaning and ridiculing his rank-and-file supporters - those forgotten men and women who aren't chanting at the rallies - an insult to him is an insult to them and vice versa," Ms Conway said.
One such Trump supporter, Laura Capps, 39, had driven last week from Boone, Iowa, to attend the first full day of the state fair. Ms Capps said she was exasperated when Democrats blamed Trump for mass shootings - "there were shootings under Obama, under every president" - and said they obsessed over Trump's tweets and statements because they had nothing else to attack.
"I've been called a racist because I'm a Trump supporter," Ms Capps said. "It's ridiculous. I've got a first cousin that's married to an African American gal. So their kids are biracial, and I love them just like the rest of my second cousins."
The Washington Post
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