Bloomberg via Getty Images Sen. Elizabeth Warren (D-Mass.) says it's time to stand up to Donald Trump.

Sen. Elizabeth Warren (D-Mass.) tore into Donald Trump on Monday, turning the Republican presidential front-runner's favorite insult against him.

However, Warren warned, it's important to take Trump seriously.

"Many of history's worst authoritarians started out as losers -- and Trump is a serious threat," she wrote. "The way I see it, it's our job to make sure he ends this campaign every bit the loser he started it."

Several hours after her Facebook post, Warren continued to make the case against Trump on Twitter:

.@RealDonaldTrump is “cool with being called an authoritarian,” isn't upset at Hitler comparisons & thinks we won’t fight back? WRONG! — Elizabeth Warren (@elizabethforma) March 21, 2016

Why is it that @realDonaldTrump can call himself the “unity” candidate while basing his campaign on racism, sexism, xenophobia & hatred? — Elizabeth Warren (@elizabethforma) March 21, 2016

We cannot elect wannabe tyrants to the White House. Not now, not ever. It’s up to all of us us to stop @realDonaldTrump . — Elizabeth Warren (@elizabethforma) March 21, 2016

While Warren has not endorsed in the Democratic primary, the progressive senator has strongly condemned Trump -- and the Republican Party for enabling his dominance in the race.

Last week, she urged "decent people everywhere" to stand up to Trump, arguing the businessman is a "bigger, uglier threat every day that goes by." And in an interview with MSNBC earlier this month, she argued the GOP only has itself to blame for Trump's rise.

"They are paying the price for their own extremism. It has now taken them by the throat," she said of the Republican Party. "Guys, this is what you did to yourselves."

UPDATE, 3:01 p.m. ET: When asked about Warren's comments, Trump referred to Warren as "the Indian," an apparent reference to a 2012 controversy over Warren's Native American heritage.

"You mean the Indian?" Trump says when asked about Warren calling him a loser. — Patrick Svitek (@PatrickSvitek) March 21, 2016

Editor's note: Donald Trump regularly incites political violence and is a serial liar, rampant xenophobe, racist, misogynist and birther who has repeatedly pledged to ban all Muslims -- 1.6 billion members of an entire religion -- from entering the U.S.

This story has been updated to include Warren's later tweets.