Donald Trump’s re-election campaign has harnessed Facebook advertising to push the idea of an “invasion” at the southern border, amplifying the fear-inducing language about immigrants that he has also voiced at campaign rallies and on Twitter.

Since January, the US president’s re-election campaign has posted more than 2,000 ads on Facebook that include the word “invasion” — part of a barrage of advertising focused on immigration, a dominant theme of his re-election messaging.

A review of Mr Trump’s tweets also found repeated references to an “invasion,” while his 2016 campaign advertising heavily featured dark warnings about immigrants breaching America’s borders.

Mr Trump’s language on immigration — particularly his use of the word “invasion” — is under scrutiny after the mass shooting in El Paso, Texas, on Saturday.

The suspect in that shooting, which left 22 people dead, appeared to be the author of a manifesto declaring that “this attack is a response to the Hispanic invasion of Texas.”

El Paso mass shooting: 20 killed at Walmart store Show all 39 1 /39 El Paso mass shooting: 20 killed at Walmart store El Paso mass shooting: 20 killed at Walmart store CCTV images of the gunman identified as Patrick Crusius The 21 year old, as he entered the Cielo Vista Walmart store in El Paso. The gunman was armed with an assault rifle and opened fire on shoppers at a packed Walmart store, killing 20. KTSM 9/AFP/Getty El Paso mass shooting: 20 killed at Walmart store Law enforcement agencies respond The Texas city’s police chief said the assault on a Walmart store on Saturday, which left another 26 people wounded, was being investigated as a potential hate crime. AFP/Getty El Paso mass shooting: 20 killed at Walmart store Shoppers exit with their hands up Shoppers exit with their hands up after a mass shooting at a Walmart in El Paso, Texas, U.S. August 3, 2019. REUTERS/Jorge Salgado NO RESALES. NO ARCHIVES. STRINGER Reuters El Paso mass shooting: 20 killed at Walmart store FBI released a picture of gunman Patrick Crusius The police officially identified the 21-year-old white male from Allen, Texas, a Dallas suburb some 650 miles east of El Paso. FBI/AP El Paso mass shooting: 20 killed at Walmart store A woman reacts after the mass shooting The attack came just minutes after a far-right manifesto appeared online. If authentic, it would make it the third mass shooting this year announced in advance on the website, which often features far-right and racist content. Reuters El Paso mass shooting: 20 killed at Walmart store Law enforcement responds to the active shooter The racist four-page document, titled “The Inconvenient Truth”, calls the Walmart attack “a response to the Hispanic invasion of Texas” and expresses support for the gunman who killed 51 people at two mosques in Christchurch, New Zealand earlier this year. AFP/Getty El Paso mass shooting: 20 killed at Walmart store A shopper hiding with an old lady behind the return and exchanges counter as the shooting began. Aaron Castaneda/Reuters El Paso mass shooting: 20 killed at Walmart store Ambulances in the car park near the scene It is ranked as the eighth-deadliest mass shooting in modern US history, after a 1984 shooting in San Ysidro, California, that claimed 21 lives. AP El Paso mass shooting: 20 killed at Walmart store Kendall Long (left) comforts Kianna Long (right) who was in the freezer section of Walmart. EPA El Paso mass shooting: 20 killed at Walmart store epa07755367 Police stand at attention after a shooting at a Walmart in El Paso, Texas, USA, 03 August 2019. According to reports, at least one person was killed and at least 18 people injured and transported to local hospitals. One suspect is in custody. EPA/IVAN PIERRE AGUIRRE IVAN PIERRE AGUIRRE EPA El Paso mass shooting: 20 killed at Walmart store Shoppers exit with their hands up. Reuters El Paso mass shooting: 20 killed at Walmart store Shopping carts sit next to a curb after the shooting. EPA El Paso mass shooting: 20 killed at Walmart store El Paso Fire Medical personnel arrive at the scene. AP El Paso mass shooting: 20 killed at Walmart store Walmart employees react after. AP El Paso mass shooting: 20 killed at Walmart store A woman runs to police near the scene. AP El Paso mass shooting: 20 killed at Walmart store Several law enforcement agencies respond. AP El Paso mass shooting: 20 killed at Walmart store People arrive at MacArthur Elementary looking for family and friends as the school is being used a re-unification centre. AP El Paso mass shooting: 20 killed at Walmart store Residents Erica Rios, 36, and Alma Rios, 61, cry outside a reunification centre. AFP/Getty El Paso mass shooting: 20 killed at Walmart store People gather in Juarez, Mexico, in a vigil for the Mexican nationals who were killed. AP El Paso mass shooting: 20 killed at Walmart store A child takes part in a vigil in Ciudad Juarez AFP/Getty El Paso mass shooting: 20 killed at Walmart store Francisco Castaneda joins mourners taking part in a vigil at El Paso High School. Reuters El Paso mass shooting: 20 killed at Walmart store Presidential candidate and former congressman Beto O'Rourke, right, meets with mass shooting survivor, Rosemary, at University Medical Centre Beto O'Rourke Facebook via AP El Paso mass shooting: 20 killed at Walmart store From left, Melody Stout, Hannah Payan, Aaliyah Alba, Sherie Gramlich and Laura Barrios comfort each other during a vigil for victims of the shooting. AP El Paso mass shooting: 20 killed at Walmart store A sign is posted near the scene Getty Images El Paso mass shooting: 20 killed at Walmart store Police cars parked below the Walmart sign block a road outside while investigating. EPA El Paso mass shooting: 20 killed at Walmart store Sherie Gramlich reacts during a vigil. AP El Paso mass shooting: 20 killed at Walmart store A man places flowers at the site Reuters El Paso mass shooting: 20 killed at Walmart store People take part in a rally against hate a day after a mass shooting at the Walmart store Reuters El Paso mass shooting: 20 killed at Walmart store Lupe Lopez holds a picture of a victim during a vigil for victims AP El Paso mass shooting: 20 killed at Walmart store People take part in a rally against hate a day after a mass shooting at a Walmart store, in El Paso, Texas, U.S. August 4, 2019. REUTERS/Jose Luis Gonzalez JOSE LUIS GONZALEZ REUTERS El Paso mass shooting: 20 killed at Walmart store Elsa Mendoza Marquez, a Mexican schoolteacher who was married and the mother of two adult children, was one of the victims Getty El Paso mass shooting: 20 killed at Walmart store People with the Mexican flag and the US flag take part in a rally against hate a day Reuters El Paso mass shooting: 20 killed at Walmart store People raise their arms in the air during a vigil for victims AP El Paso mass shooting: 20 killed at Walmart store People react and embrace each other Getty El Paso mass shooting: 20 killed at Walmart store Women light candles at a make shift memorial at the site of a mass shooting EPA El Paso mass shooting: 20 killed at Walmart store Adria Gonzalez (centre) who is being hailed as a hero for leading some Walmart customers to safety, speaks to the crowd AFP/Getty El Paso mass shooting: 20 killed at Walmart store People hold up their phones AFP/Getty Images El Paso mass shooting: 20 killed at Walmart store Beto O'Rourke speaks to the crowd AFP El Paso mass shooting: 20 killed at Walmart store People react during a prayer and candle vigil organized by the city, after a shooting left 20 people dead at the Cielo Vista Mall WalMart in El Paso, Texas, on August 4, 2019. - A shooting at a Walmart store in Texas left multiple people dead. At least one suspect was taken into custody after the shooting in the border city of El Paso, triggering fear and panic among weekend shoppers as well as widespread condemnation. It was the second fatal shooting in less than a week at a Walmart store in the US and comes after a mass shooting in California last weekend. (Photo by Mark RALSTON / AFP)MARK RALSTON/AFP/Getty Images MARK RALSTON AFP/Getty

Divisive and often false claims about unauthorised immigrants have been a cornerstone of Mr Trump’s political strategy for years, from the “build the wall” chants at his 2016 campaign rallies to his warnings about a migrant caravan before the 2018 midterm elections.

In his re-election campaign, Mr Trump has spent an estimated $1.25m (£1m) on Facebook ads about immigration since late March, according to data from Bully Pulpit Interactive, a Democratic communications firm that is tracking the digital political advertising of presidential candidates.

Those ads represent a significant portion of the roughly $5.6m (£4.6m) that Trump has spent on Facebook advertising during that period.

Most of the “invasion” ads began running between January and March, though a few dozen began running in May. Many of the ads began with a blunt message — “We have an INVASION!” — and went on to say, “It’s CRITICAL that we STOP THE INVASION.”

Mr Trump’s campaign, like other advertisers, runs many different Facebook ads using text or visuals that vary. Ads with the word “invasion” make up a small portion of the ads the Trump campaign has run on Facebook this year.

Facebook’s archive of political ads, which dates back to May 2018, says it contains more than 240,000 Trump ads.

There is no evidence that Mr Trump’s Facebook ads directly influenced the author of the manifesto, who wrote that his views “predate Trump” and posted the document on 8chan, an online forum known as a haven for extremists.

But Mr Trump, through his speeches, tweets and campaign ads, has elevated the idea of an “invasion,” once a fringe view often espoused by white nationalists, into the public discourse.

Some other Republican candidates have echoed Mr Trump’s language in their own ads.

“Let’s call this what it is — an invasion of our country,” read a recent Facebook ad for Tommy Tuberville, a former Auburn football coach who is running for Senate in Alabama.

A young man prays while attending the make shift memorial along the street after the mass shooting that happened at a Walmart in El Paso (EPA)

Other Republicans who have used the word “invasion” in Facebook ads include a candidate for governor in West Virginia and a candidate for Senate in North Carolina.

Cognitive linguist George Lakoff said the word “invasion” was a potent one for Mr Trump to use because of what it allowed him to communicate.

“If you’re invaded, you’re invaded by an enemy,” he said. “An invasion says that you can be taken over inside your own country and harmed, and that you can be ruled by people from the outside.”

Mr Lakoff added: “When he’s saying ‘invasion,’ he’s saying all of those things. But they’re unconscious. They’re automatic. They’re built into the word ‘invasion’.”

For the writer of the manifesto, the concept of an “invasion” had an additional, racist meaning: He promoted a conspiracy theory called “the great replacement,” which claims that an effort is underway to replace white people with nonwhite people.

Democratic candidates for president blamed Mr Trump for helping spread such views.

“White supremacy is not a mental illness,” Elizabeth Warren, senator of Massachusetts, said on Monday. “We need to call it what it is: Domestic terrorism. And we need to call out Donald Trump for amplifying these deadly ideologies.”

But radio host Rush Limbaugh attacked Democrats and the news media on Monday for pointing the finger at conservatives like him.

“We’re sick and tired, every time this happens, people that we believe in being blamed for it,” he said. “We’re sick of it. None of us pulled the trigger, none of us want these things to happen, and yet we turn on the media and that’s what we hear.”

The Mexican and US flags fluttered alongside each other as thousands in the southern of border city of El Paso united as one (AFP/Getty Images)

Stoking fear about immigrants has been central to the Trump campaign’s advertising strategy since it first began airing political commercials during the 2016 race.

The campaign’s first ad of that election focused on “radical Islamic terrorism” in the wake of the mass shooting in San Bernardino, California, and showed footage of people seemingly flooding across a border. (The footage was from Morocco, not the United States.)

Mr Trump also proposed a “total and complete shutdown of Muslims entering the United States” after the attack.

Scenes evoking illegal immigration became common during the 2016 effort, and Mr Trump painted a picture of an America overwhelmed by immigrants.

“We don’t have a country right now,” he said in footage shown in one ad. “We have people pouring in, they’re pouring in, and they’re doing tremendous damage.”

The use of alarmist language and imagery about immigrants has a history in the modern Republican Party that dates back to the divisive political battles over illegal immigration in the 1990s.

One of the most infamous depictions of migrants as a threat came from a 1994 ad from Pete Wilson, governor of California, that showed a group of people rushing through a border crossing. “They keep coming,” the announcer said.

Since then, images of shadowy figures climbing fences or prowling around in the dark have been a staple of Republican campaign ads, often used by candidates whose districts and states are far from any border.

In 2014, for example, Pat Roberts, senator of Kansas, ran an ad that was typical of the Republican messaging at the time, warning of “a border crisis” that was “taking jobs away from Kansans”.

Mr Trump’s takeover of the party gave those kinds of messages a higher platform and a larger mouthpiece as conservative media outlets like Fox News amplified his words.

He seized on the “invasion” imagery in the run-up to the 2018 midterm elections, when he claimed without evidence that a caravan of migrants making its way north towards the border had been infiltrated by “criminals and unknown Middle Easterners”.

'This will only make it stronger': El Paso residents react to shooting

The president and fellow Republicans warned of waves of violence, drugs and crime that awaited the country if it were led by Democrats, who were portrayed as supporting policies that would weaken national security. That effort did not have the desired effect, as Republicans lost control of the House.

Customs and Border Protection recorded more than 144,000 arrests at the southwestern border in May, the highest monthly total in 13 years. Arrests declined by 28 per cent in June and were expected to continue to fall through July, according to officials in the Department of Homeland Security.

The estimated $1.25m (£1m) that the Trump campaign has spent on immigration ads on Facebook since late March is a significant sum when compared with the amount of money the Democratic presidential candidates are spending on Facebook ads. The spending figures include both Facebook and Instagram.

Over the same period, no Democratic candidate spent more than $2.1m (£1.7m) on Facebook ads, according to Bully Pulpit; only Kirsten Gillibrand, senator of New York, and former Joe Biden, the former vice president, topped $2m (£1.6m) through Saturday.

Mr Trump’s spending on immigration ads exceeded the entirety of what one of the best-funded Democrats — Kamala Harris, senator of California — spent on Facebook during that period.