Donald TrumpDonald John Trump Former US ambassador: 'Denmark is not a big fan of Donald Trump and his politics' Senate Democrats push for arms control language in defense policy bill Detroit county sheriff endorses Booker for president MORE on Monday said that calling Sen. Elizabeth WarrenElizabeth Ann WarrenSenate Democrats push for arms control language in defense policy bill Top adviser on Sanders: 'He's always been underestimated' 'The Simpsons' pokes fun at Trump's feud with 'the squad' MORE (D-Mass.) “Pocahontas” is only demeaning to the renowned Native American.

“I do regret calling her Pocahontas, because I think it’s a tremendous insult to Pocahontas,” he said on Fox News’s “The O’Reilly Factor" Monday night. "So, to Pocahontas, I would like to apologize to you.”

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Anchor Bill O’Reilly also asked Trump, the GOP’s presumptive presidential nominee, whether he would consider Warren for “ambassador to Tonga.”

“No, she wouldn’t be my first choice,” Trump responded. "You know, she’s been a very, very poor senator. You know the funny thing about Elizabeth Warren, she’s been a lousy senator. She’s done a terrible job.”

Trump has repeatedly dubbed Warren “Pocahontas,” reviving past controversy over the senator’s claims of Native American heritage.

Warren has frequently sparred with Trump in recent weeks, attacking the billionaire’s background and disparaging remarks he has made about women.

Warren last week visited Hillary ClintonHillary Diane Rodham ClintonThe Memo: Polls flash warning signs for Trump Polls suggest Sanders may be underestimated 10 declassified Russia collusion revelations that could rock Washington this fall MORE’s presidential campaign headquarters in New York City, fueling speculation she may be a vice presidential pick.

Clinton, the presumptive Democratic presidential nominee, was not present during Warren’s appearance.

Warren reportedly discussed protecting ObamaCare and the Dodd-Frank financial reform law during her visit.

Warren’s stop followed her endorsement of Clinton earlier in June after months of neutrality in the Democratic presidential primary.

“I’m ready to get in this fight and work my heart out for Hillary Clinton to become the next president of the United States and make sure Donald Trump never gets anywhere near the White House,” she said on MSNBC on June 9.

Clinton leads Trump by 6 points nationwide, according to the latest RealClearPolitics average of polls.

Trump on Monday said he would close the gap in voter popularity separating him from Clinton before this November.

“I think it’s very close,” he said of the general presidential election. "It’s fairly early to have polling. I think we haven’t even gotten started yet. But I think it’s pretty even from what I’m seeing."