Sen. Elizabeth Warren of Massachusetts, who many liberals hoped would throw her hat in the 2016 presidential race, is speaking out against Donald Trump.

The Massachusetts politician penned a string of fiery tweets on Monday, calling Trump a “loser” whose “insecurities are on parade.” She criticized the Republican frontrunner for racism, narcissism and denigrating comments about women.

Warren also compared Trump to “many of history’s worst authoritarians” and warned voters that the businessman is a “serious threat.”

Trump won the early March Republican primary in Warren’s home state of Massachusetts with 49% of the vote and appears on track to do well in the upcoming Arizona primary, though he’s expected to lose in Utah at the hands of Mormon voters.

The tweets were sent from Warren’s campaign-affiliated account. A press officer for Warren told TIME the Senator writes her own tweets.

Trump did not immediately respond to Warren’s criticisms on Twitter.

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