Bergen, the professor of Holocaust studies, said that the use of the red triangle was most pernicious not simply because it harkens back to Nazism and concentration camps, but because of the moment in Nazi history it represents.

Most Jews are familiar with the yellow star patches that Nazis forced Jews to sew into their clothes. But those were not required in Germany until 1941.

When Hitler moved to seize dictatorial power in Germany in February 1933, the first people arrested en masse were communists, suspected communists and their supporters, she said. They were rounded up in the first concentration camp -- Dachau -- and labeled with the red triangle. With the communist members of the German parliament imprisoned, Hitler's legislative allies were able to pass a law that gave him near-complete control of what had until recently been a free German society.

"It's a political symbol of a means that was used to crush a democratic process," Bergen said of the triangle.

Allegations that Trump's presidential campaign has used images tied to anti-Semitic conspiracies and Nazism have dogged him since 2015. In July of that year, he ran an ad that used a stock photograph of Nazi soldier re-enactors in a place that many would have expected to see American soldiers: layered under the American flag. Trump said the image was the fault of a "young intern."

In 2016, Trump tweeted an image of Hillary Clinton's face superimposed over a pile of money with a six-pointed star -- often identified by Jews as a Star of David -- that contained the words "Most Corrupt Candidate Ever!" The graphic had been shared on a Nazi internet forum, and critics considered it a "dog whistle" to white supremacist supporters. Trump defended the tweet by saying that the star could have been a Sheriff's star or a "plain star." He later used a poster of graphic earlier shared by David Duke, a former Klu Klux Klan leader and anti-Semite.

More examples have cropped up during his presidency. In August 2019, his official Twitter account posted a video supporting his reelection campaign that featured a stylized lion icon, one tied to the white nationalist website VDARE and white supremacist groups.

In May, he praised the "good bloodlines" of Henry Ford, a notorious anti-Semite with strong ties to the Nazi regime.

Last week Trump referred to the Secret Service as the "S.S.", which one Jewish group deemed a "dog whistle to white supremacists and neo-Nazis" for its apparent reference to the Nazi paramilitary police responsible for enforcing the regime's racial policies.

In response to the triangle ad, Bend the Arc, a Jewish social justice group, noted that it was not an anomaly.

"This is dozens of carefully targeted ads from the official pages of Mike Pence, Donald Trump, and Team Trump," the group said in a tweet. "All paid for by Trump and the Republican National Committee. All spreading lies and genocidal imagery."