Associations of antisemitism returned to shadow Donald Trump’s presidential campaign on Sunday, after the Republican and his allies, including a three-star general who was a vice-presidential hopeful, were forced again to answer questions about supporters with explicitly bigoted views.

The retired army lieutenant general Michael Flynn retweeted an explicitly antisemitic message regarding the leak of thousands of emails from the Democratic National Committee.

Flynn, who was on the shortlist to be Trump’s VP and spoke during primetime at the Republican national convention last week, retweeted a post in sympathy with a Trump supporter who mocked Hillary Clinton’s campaign, which has blamed Russian hackers for the leak.

The pseudonymous Trump supporter tweeted: “CNN implicated. ‘The USSR is to blame!’ Not anymore, Jews. Not anymore.”

The retired general deleted his retweet and later said “the earlier retweet was a mistake” and that he had meant to link to an article on Clinton and the DNC emails. “My sincerest apologies,” he added.

On Saturday, Trump hinted that he would consider Flynn for secretary of defense, although he is ineligible under federal law, which requires candidates to have spent at least seven years out of active service.

A spokesperson for the Republican National Committee told the Guardian: “There is no place in the Republican Party for antisemitic comments.”

In an interview broadcast on Sunday, Trump had once again to answer questions about one of his loudest and most notorious supporters: David Duke, a former grand wizard of the Ku Klux Klan who on Friday announced a run for Senate in Louisiana.

Flynn’s tweet. Photograph: Screengrab

“I’m overjoyed to see Donald Trump and most Americans embrace most of the issues that I’ve championed for years,” Duke said, a day after praising Trump’s platform as reflective of his own.

In February, Trump wavered on disavowing Duke’s endorsement, claiming “I know nothing about white supremacists” even though he had named Duke “a Klansman” in 2000.

On Sunday, the Republican nominee was much faster to reject Duke, saying he would potentially support a Democrat against him. “Depending on who the Democrat [were],” Trump told NBC’s Meet the Press. “But the answer would be yes.”

The candidate added: “Rebuked. Is that OK? Rebuked, done.”

Duke has remained unfazed by Trump’s disavowal, in June defending the businessman from accusations of racism by saying they were “very illustrative of the Jewish tribal nature”, and that Jewish people behaved “like a pack of wild dogs”.

The party’s chairman, Reince Priebus, has forcefully rebuked any hint of association with antisemitism, saying on Saturday: “David Duke [and] his hateful bigotry have no place in the Republican party and the RNC will never support his candidacy under any circumstance.”

Trump himself was accused of antisemitism earlier this month, when his campaign tweeted an image of Clinton juxtaposed with images of money, a six-pointed star and the words “most corrupt candidate ever”. The tweet was deleted, but Trump defended it and insisted it did not show a star of David, even though the image was traced to an account the regularly posts antisemitic material.

The businessman has also adopted the rallying cry “America First”, which became famous as a slogan of the famous aviator Charles Lindbergh and isolationists in the 1930s and early 1940s. Lindbergh, who was welcomed in Nazi Germany several times before the second world war, wrote about “racial strength” and said civilization depended “on a western wall of race and arms which can hold back either a Genghis Khan or the infiltration of inferior blood”.

The Anti-Defamation League asked Trump to stop using the phrase in April, but it appeared prominently throughout the Republican convention. Trump told the New York Times last week that he considers the phrase “a brand-new, modern term”.

Jewish reporters have suffered a barrage of antisemitic harassment from Trump supporters for coverage deemed negative toward the candidate. The businessman has tried to appeal to Jewish Republicans by noting that his daughter and son-in-law are Jewish.

In December he told the Republican Jewish Coalition: “Look, I’m a negotiator like you folks, we’re negotiators.”

He added: “Is there anybody who doesn’t negotiate deals in this room?”