Former U.S. President Donald Trump, now the Republican presidential frontrunner, said on Saturday that undocumented immigrants were "poisoning the blood of our country," repeating language that has previously drawn criticism as xenophobic and echoing of Nazi rhetoric.

Trump made the comments during a campaign event in New Hampshire where he railed against the record number of migrants attempting to cross the U.S. border illegally. Trump has promised to crack down on illegal immigration and restrict legal immigration if elected to a second four-year term in office.

"They're poisoning the blood of our country," Trump told a rally in the city of Durham attended by several thousand supporters, adding that immigrants were coming to the U.S. from Asia and Africa, in addition to South America. "All over the world they are pouring into our country."

Trump used the same "poisoning the blood" language during an interview with The National Pulse, a right-leaning website, that was published in late September. It prompted a rebuke from the Anti-Defamation League, whose leader, Jonathan Greenblatt, called the language "racist, xenophobic and despicable."

Jason Stanley, a Yale professor and author of a book on fascism, said Trump's repeated use of that language was dangerous. He said Trump's words echoed the rhetoric of Nazi leader Adolf Hitler, who warned against German blood being poisoned by Jews in his political treatise "Mein Kampf".

"He is now employing this vocabulary in repetition in rallies. Repeating dangerous speech increases its normalization and the practices it recommends," Stanley said. "This is very concerning talk for the safety of immigrants in the U.S."

Trump is the leading candidate for the Republican Party's 2024 presidential nomination and has made border security a major theme of his campaign. He is vowing to restore the hardline policies from his 2017-2021 presidency, and implement new ones that clamp down further on immigration.

U.S. President Joe Biden, the presumptive Democratic Party nominee, has struggled with record levels of migrants, a problem seen as a vulnerability for his re-election campaign.

On the campaign trail, Trump has repeatedly used inflammatory language to describe the border issue and slam Biden's immigration policies. On Saturday he recited the lyrics of a song he has repurposed to liken immigrants to deadly snakes.

If re-elected, Trump promised "to stop the invasion of our southern border and begin the largest domestic deportation operation in American history."

(With input from Reuters)