Donald Trump used a racist article by a conservative commentator he once described as a “Hitler lover” who “doesn’t like the blacks” to justify his border wall.

The US president approvingly quoted an opinion piece in which Pat Buchanan warned against a “multiracial, multiethnic, multicultural, multilingual America”.

Mr Buchanan, a former aide to Richard Nixon and Ronald Reagan, claimed Democrats opposed a wall on the US-Mexico border because they wanted “to increase the number of women, African-Americans, Asians and Hispanics, and thereby reduce the number of white men”.

“The Democratic Party is hostile to white men, because the smaller the share of the US population that white men become, the sooner that Democrats inherit the national estate,” he suggested.

In tweets posted late on Sunday night, Mr Trump quoted a section of the article in which Mr Buchanan wrote: “America’s southern border is eventually going to be militarized and defended or the United States, as we have known it, is going to cease to exist.”

The president added: “The great people of our Country demand proper Border Security NOW!”

Footage soon surfaced on social media of a 1999 interview with NBC News in which Mr Trump attacked Mr Buchanan, who was vying against him for the Reform Party’s presidential nomination at the time.

Mr Trump told Meet the Press host Tim Russert: “He’s a Hitler lover. I guess he’s an anti-Semite. He doesn’t like the blacks, he doesn’t like the gays.

“It’s just incredible that anybody could embrace this guy.”

About Mr Buchanan’s bid for the White House, he added: “Maybe he’ll get four or five per cent of the vote, and it’ll be a really staunch right wacko vote. I’m not even sure if it’s right. It’s just a wacko vote. And I just can’t imagine that anybody can take him seriously.”

Mr Buchanan has previously been accused of antisemitism and Holocaust denial. He wrote in a New York Post column in 1990 that it would have been impossible for 850,000 Jews to be murdered by a diesel engine exhaust fed into a gas chamber at the Nazi death camp in Treblinka.

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The Anti-Defamation League has described Mr Buchanan as an “unrepentant bigot” with views “identical to those of self-declared white nationalists”.

Mr Trump’s embracing of the former political rival he once strongly denounced comes as he seeks to justify his demand for $5.6bn funding for the border wall, a key promise of his 2016 election campaign.

The president's rhetoric on the issue has hardened as his dispute with Democrats over the issue prolongs a government shutdown that at 24 days is now the longest in US history.

Ted Lieu, the Democrat Representative for California, tweeted in response to Mr Trump’s latest posts: ”Hey @realDonaldTrump: Unless you retract your tweet promoting this racist Pat Buchanan article, very difficult now for any Democrat to support your wall. Ever.”