As Trump tweeted a clip on Wednesday stoking panic about migrants, the 2018 elections have revealed a surge in extremist rhetoric that passes for political campaigning

Is this the most racist US midterms campaign ever?

In the US, political advertising is often laced with venom.

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From the Nixon-Kennedy attack ads of 1960 to the Romney-Obama clashes of 2012 this unique, aggressive form of campaigning has a long history, protected by the first amendment. But rarely, if ever, has a president endorsed advertising as nakedly divisive and overtly prejudiced as has been seen this year.

This week, Donald Trump tweeted an incendiary video that stoked panic about migrants and falsely claimed Democrats were responsible for the murder of two police officers in California.

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The 53-second video was produced by a consultancy firm in Washington responsible for other rightwing ads. It depicts Luis Bracamontes, who was in the US illegally when he murdered the officers in Sacramento in 2014. Over courtroom footage in which the perpetrator brags about the murders, the video states: “Illegal immigrant Luis Bracamontes killed our people! ... Democrats let him into our country ... Democrats let him stay.”

Though news organisations were quick to point out that the ad was inaccurate, it was not new to say the president had endorsed false and racist comments about immigration. The billionaire launched his political career in 2015 by calling Mexicans migrants rapists and criminals. The 2018 midterm elections have revealed an extraordinary proliferation in the sort of extremist rhetoric that now passes for political campaigning.

The fact that the leader of the free world is the liar of the free world is having an effect Rick Wilson

“The fact that the leader of the free world is the liar of the free world is having an effect,” said Republican strategist and media consultant Rick Wilson.

“The difference we’re experiencing right now is twofold. One, we live in a post-truth environment because of Trump, so there is zero concern or consideration whether or not any of these things that [the adverts] are actually saying or not are actually true …

“[Secondly,] there’s been a long, slow collapse of trust in public institutions, in particular political institutions, and that is something that poisoned the atmosphere for a long time before Donald Trump came along.”

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In the Senate race in Tennessee, Tea Party Republican and longtime Trump supporter Marsha Blackburn faces Democrat Phil Bredesen, a former governor. Last week, Blackburn released an attack ad littered with false information about a group of migrants who are slowly approaching the US-Mexico border.

Echoing Trump, the ad said the group was full of “gang members … known criminals … people from the Middle East … possibly terrorists”. It accused Bredesen of endangering national security.

In Georgia, a closely-fought gubernatorial race pits Republican Brian Kemp against Democrat Stacey Abrams, who would become the first African American female governor in US history. Kemp, a staunch Trump supporter, has released videos in which he poses with his pickup truck and claims: “I’ve got a big truck just in case I need to round-up criminal illegals and take them home myself.”

Pro-Kemp ads have falsely accused Abrams of trying to get undocumented migrants to vote.

Other ads have gone after candidates on grounds of race or religion. In New York, the National Republican Congressional Committee released an ad attacking the Democrat Antonio Delgado, an African American Rhodes scholar who released a rap album in 2007. Referred to as a “big city rapper”, Delgado was described as “not like us”.

Other ads are more direct. In California, Ammar Campa-Najjar, a Latino-Arab American who is Christian, has been falsely accused of having ties with the Muslim Brotherhood. One Republican ad called him a “security risk”.

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Wilson knows of what he speaks, having been responsible for a number of highly charged Republican ads himself. In 2002, as media adviser for Saxby Chambliss’s successful Senate run in Georgia, he released an ad that attacked the Democratic candidate’s national security record while showing images of Saddam Hussein and Osama bin Laden. The video caused significant controversy. Wilson defended it.

“Those ads were based in facts and it made them more powerful rather than less,” he said. “They were part of a contextual moment of the time on a national issue. They were also designed to break off voters in the middle to come in your direction.”

Recent research into attack ads used in US elections in 2010 and 2012 indicates they continued to be an effective tool in persuading undecided voters and driving turnout. But experts like Wilson – the author of the book Everything Trump Touches Dies – remain unconvinced that a proliferation of demonstrably false and prejudiced attacks will do much to convince those still on the fence.

“All the Trump adverts now are just meant to stoke his base, and his base only,” he said. “There’s no persuasion outside of the base now for him. It’s all about the cadre, the core voter.”

Tuesday’s midterms are sure to put that theory to the test.