Elizabeth Warren. Chip Somodevilla/Getty Images Elizabeth Warren took Republican presidential candidate Donald Trump to task Monday over his business record and provocative campaign-trail comments.

The Massachusetts senator slammed Trump in a series of tweets, repeatedly calling him a "loser" while dismissing his "scam" businesses.

"Let's be honest - @realDonaldTrump is a loser. Count all his failed businesses. See how he cheated people w/ scams like Trump U," Warren said, referring to Trump University, the real-estate magnate's embattled for-profit college.

She posted a similar, longer message on Facebook as well.

View Warren's tweets below:

But Warren also warned of the danger of not taking Trump's candidacy seriously.