Republican presidential candidate Donald Trump referred to Massachusetts Sen. Elizabeth Warren as "the Indian" at a news conference on Monday just hours after she unleashed a Twitter tirade against him."Who is that, the Indian? You mean the Indian?" Trump asked, referring to Warren, who has said she has Native American ancestry.Warren's contention became a point of controversy during her 2012 U.S. Senate race when her Republican opponent, former Massachusetts Sen. Scott Brown, who demanded proof. Warren has repeatedly defended her statements.The Republican frontrunner was asked by reporters about the Democratic senator's attacks during the event at the Trump International Hotel in Washington D.C.This is not the first time Trump has invoked Warren's heritage. He did so in an interview withcolumnist Maureen Dowd published over the weekend."I think it's wonderful because the Indians can now partake in the future of the country," Dowd quoted Trump as saying. "She's got about as much Indian blood as I have."Warren took on Trump earlier today by giving the GOP presidential front-runner a taste of his own medicine, using social media and taunts often employed by the man himself.The New York real estate mogul is a "loser," Warren tweeted today, also calling him "a serious threat."Warren is considered by many to be the most sought-after endorsement in the Democratic Party , other than President Obama, but she has not endorsed Hillary Clinton or Bernie Sanders in the 2016 campaign.Every other female Democratic senator has thrown her support behind Clinton.