Image copyright Getty Images Image caption US border at Tijuana, Mexico

Donald Trump is once again using Twitter to weigh in on contentious religious-tinged political issues in the UK.

In the past, he's attacked London's mayor, accusing him of downplaying a militant attack just hours after it occurred.

"Pathetic excuse by London Mayor Sadiq Khan who had to think fast on his 'no reason to be alarmed' statement," he tweeted after the 5 June London Bridge attack.

He misattributed a rise in crime in England and Wales to the "spread of radical Islamic terror".

"Not good, we must keep America safe!" he wrote.

Now, he has retweeted a series of unverified videos posted by a far-right British nationalist group.

For the president, directing attention toward the UK seems to serve a domestic political purpose - the "keep America safe" line from his English crime tweet is telling.

He cites events and opinions there as a warning to Americans of what could happen in the US if they do not heed his policy prescriptions on immigration and border security.

The Muslim ban, the US-Mexico wall, increased deportations, the sharp reductions on refugee resettlement - it's all part of the president's "national security" package.

It's what Mr Trump campaigned on, and the aggressiveness of his rhetoric was one of the ways he differentiated himself from his opponents.

The president's base firmly believes that conditions in the UK, and throughout Europe, are deteriorating in large part due to their immigration policies, and have stuck with Mr Trump through the bumps in his presidential campaign and a tumultuous first year as president in part because of his convictions on the matter.

Media playback is unsupported on your device Media caption The White House says Trump's Britain First tweets were about border security

When pressed by reporters outside the White House, press secretary Sarah Huckabee Sanders reinforced this point, downplaying concern about the authenticity of the videos.

"Whether it's a real video, the threat is real and that is what the president is talking about that is what the president is focused on is dealing with those real threats ," she said.

While most Americans probably haven't heard of the Britain First group that originally posted the videos, and are unfamiliar with European radical nationalist movements, there are white supremacist groups in the US that follow the actions of these overseas operations quite closely.

The president on Wednesday signalled that he watches them, too.

As the controversy continued to boil, former Ku Klux Klan leader David Duke took to Twitter to express his support for the president.

"He's condemned for showing us what the fake news media WON'T," the former Louisiana political candidate wrote. "Thank God for Trump! That's why we love him!"

Image copyright Getty Images

While Trump's tweets regularly generate a flurry of media coverage, concern and - sometimes - condemnation, his retweets, uncommented upon, but distributed to his millions of followers, often have their own controversial story to tell.

In August the president retweeted a cartoon of a Trump-emblazoned train smashing into man covered by a CNN logo just days after a white nationalist in Charlottesville struck and killed a counter-protester with his car. (He would later delete the post.)

During the president campaign, Mr Trump also opened himself up to allegations of anti-Semitism when he retweeted a graphic containing a photo of Hillary Clinton on a field of US currency next to a Jewish Star of David with the words "Most Corrupt Candidate Ever" written on it.

Candidate Trump also retweeted posts by an account called "@WhiteGenocideTM" twice in 2016.

There's a common refrain on Twitter that "retweets don't equal endorsements". When those retweets come from the president of the United States, and they gain an audience of millions and drive news cycles for days, that becomes a difficult assertion to make.