Donald Trump, the presumptive Republican presidential nominees, ramped up his rhetoric against Massachusetts Senator Elizabeth Warren, calling her a “racist” who “pretended to be a Native American in order to advance her career.” In his remarks, made late Saturday on Twitter, Trump referred to Warren as “Pocahontas” — a reference to the ongoing controversy regarding Warren’s ancestry.

Trump and Warren — one of his most vocal critics — have traded barbs through the week. While Trump has repeatedly referred to Warren as Pocahontas — on one occasion eliciting Native American war whoops from a crowd of his supporters — Warren has slammed the “small, insecure” billionaire for comments made in 2006, when he claimed to be “excited” by a looming market downturn that would cause real estate prices to plunge, and for his “racist, sexist” remarks.

“To root for people to lose their pensions, to root for two little girls in Clark County Nevada to end up living out of a van - what kind of a man does that? I'll tell you exactly what kind of a man does that. It's a man who cares only about himself. A small, insecure money-grubber who doesn't care who gets hurt as long as he makes a profit,” Warren said last month during a speech in Washington.

Earlier, on Thursday, Warren endorsed Hillary Clinton for president, further fuelling speculation that she may be named her running mate.

“I’m ready to get into this fight to make Hillary Clinton the president of the United States and be sure that Donald Trump gets nowhere near the White House,” Warren said during MSNBC’s “Rachel Maddow Show” Thursday evening.

Trump’s latest attack on Warren comes just days after the latter called Trump’s comments about the Indiana-born federal judge Gonzalo Curiel — who is currently hearing a case against Trump University — racist. Trump has consistently accused the judge of being biased because of his Mexican heritage.

“Like all federal judges, Judge Curiel is bound by the federal code of judicial ethics not to respond to these attacks,” Warren said Thursday. “Trump is picking on someone who is ethically bound not to defend himself, exactly what you would expect from a thin-skinned, racist bully.”