Donald Trump has an endless and varied supply of insults and playground taunts to hurl at opponents but appears to have a one-track mind when it comes to African Americans and women.

An analysis of the Trump Twitter Archive website shows that the US president has used the words “dumb” or “dummy” 236 times since 2011, and on 14 occasions since becoming taking office in January last year. He has targeted men and women, black and white.

But whereas the likes of James Comey, John McCain and Mitt Romney receive a smorgasbord of other insults, the congresswoman Maxine Waters and TV host Don Lemon, both of whom are African American, appear to be denigrated for their intelligence alone.

“Trump’s made the same criticism of black athletes, black journalists and black Members of Congress,” Peter Wehner, a veteran of three Republican administrations, tweeted last week. “He attacks their intelligence. His racist appeals aren’t even disguised anymore.”

Trump has reportedly referred to Waters as “low IQ” seven times already this year, often at high-profile campaign rallies, most recently in Ohio, where he called her “a seriously low-IQ person” before a cheering crowd. He has had little to say about other aspects of Waters’ personality or policies.

To be sure, it is not the first time he has mocked individuals’ intelligence quotient. When the Hollywood actor Robert De Niro declared “Fuck Trump!” at the Tony awards in June, the president gave a one-off response: “Robert De Niro, a very Low IQ individual, has received to many shots to the head by real boxers in movies.”

In 2015 he also wondered why the Washington Post has “low IQ people” and suggested that Governor Rick Perry of Texas “should be forced to take an IQ test”.

Perry is now energy secretary.

But his attack on Waters’ IQ is by far his most sustained. Michael Cornfield, associate professor of political management at George Washington University in Washington, said: “The strategic value is obvious: Waters is a quadruple-category demon to please Trump’s base, being black, female, leftist and aggressive in her own rhetoric.

“But it is short-term and high risk. Should the Democrats take control of the House, Waters is first in line to become chair of the financial services committee. She’ll subpoena his financial records before the sun goes down on the first day of the new session.”

The Morning Joe co-host Mika Brzezinski has been consistently targeted by Trump. Photograph: Chip Somodevilla/Getty Images

As for “dumb”, a word that seldom passed the lips of past presidents, it is clearly one of Trump’s favourites. Five years ago, long before his attack on Lemon, he anointed the TV host and comedian Bill Maher “the dumbest man on television”. Earlier this year he wrote of the former FBI director James Comey: “He’s either very sick or very dumb.” He has come up with myriad other insults for Comey, often not related to his mental capacity.

Yet Trump has now called Lemon “dumb” three times, most recently last week, when he also took a swipe at an African American basketball player: “Lebron James was just interviewed by the dumbest man on television, Don Lemon. He made Lebron look smart, which isn’t easy to do. I like Mike!” The controversial post has been retweeted nearly 50,000 times, far more than any other Trump use of “dumb”.

The CNN journalist responded on air: “Referring to African Americans as dumb is one of the oldest canards of America’s racist past and present: that black people are of inferior intelligence. This president constantly denigrates people of color and women.”

Trump’s attacks on African Americans come in the context of his past as a proponent of the “birther” movement, questioning whether Barack Obama was born in the US, his moral equivocations over white supremacist violence in Charlottesville, Virginia, a year ago and the dominance of white men in his political and judicial appointments.

It’s enough to make me nostalgic for the profanities and slurs of Richard Nixon and the vulgarities of LBJ Michael Cornfield

The New York Times columnist Charles Blow, who is African American, wrote last week: “I believe that the fact that he so often attacks the intellectual capacity of women and minorities exposes a racial and gender bias, one that has a long history and a wide acceptance.”

Indeed, Trump has reserved some of his most personal, gratuitous and outrageous insults for women. He wrote of Kirsten Gillibrand, Democratic senator for New York: “Someone who would come to my office ‘begging’ for campaign contributions not so long ago (and would do anything for them)”. He said of the then Fox News host Megyn Kelly there was “blood coming out of her wherever”.

The most powerful man in the world has labelled the broadcaster and author Mika Brzezinski “dumb as a rock”, “Crazy”, “low I.Q.”, “bleeding badly from a face-lift”, “had a mental breakdown while talking about me”, “crazy and very dumb”, “very insecure”, “not very bright”, “neurotic” and “wild with hate”.

Trump’s use of language, and limited vocabulary, is one more way in which he is upending presidential norms. Cornfield lamented: “Dummy. Dumb. It’s enough to make me nostalgic for the profanities and slurs of Richard Nixon and the vulgarities of LBJ [Lyndon Baines Johnson].”

But as Trump’s rallies testify, it appears to connect with the Trump base. Professor David Crystal, a UK-based linguist and author of The Cambridge Encyclopedia of Language, said via email: “What’s been interesting is to hear a mode of speech we associate with informal domestic or street exchange being used by someone in high office. That presumably is what made Trump so appealing to so many, avoiding the high oratory of his predecessor. ‘He talks like us,’ I’ve heard people say.”

