Donald Trump dares Hillary Clinton to pick Elizabeth Warren as her running mate

Presumptive Republican presidential nominee Donald Trump began Friday morning by taunting Hillary Clinton and Elizabeth Warren on Twitter, daring the presumptive Democratic nominee to pick the Massachusetts senator as her running mate.

Warren has already made clear that she's not afraid of going after Trump, and she said Thursday she's prepared to serve as vice president if she was chosen as Clinton's running mate.

"Pocahontas is at it again!" Trump tweeted in apparent response Friday. "Goofy Elizabeth Warren, one of the least productive U.S. Senators, has a nasty mouth. Hope she is V.P. choice."

Warren endorsed Clinton for president on MSNBC on Thursday.

While she's considered serving as Clinton's running mate, she reportedly has not discussed the idea with Clinton, sources familiar with Warren's thinking told Reuters.

But Warren has been arguably the most effective critic of Trump on the Democratic side. She blasted the brash billionaire on Twitter several times last month, and she gave a fiery speech on Thursday in which she called Trump a "loud, nasty, thin-skinned fraud," and a "racist bully."

She lobbed a fresh round of insults at Trump during her MSNBC appearance.

"Donald Trump is a genuine threat to this country," she told Maddow. "He is a threat to who we are as a people."

On Friday morning, she responded to Trump's tweet by echoing Clinton's sentiments from the day before.

"No, seriously -- Delete your account," she told Trump.

Mark Abadi contributed to this report.