Sen. Elizabeth Warren Elizabeth WarrenNo new taxes for the ultra rich — fix bad tax policy instead Democrats back away from quick reversal of Trump tax cuts It's time for newspapers to stop endorsing presidential candidates MORE (D-Mass.) blasted President-Elect Donald Trump Donald John TrumpBarr criticizes DOJ in speech declaring all agency power 'is invested in the attorney general' Military leaders asked about using heat ray on protesters outside White House: report Powell warns failure to reach COVID-19 deal could 'scar and damage' economy MORE on Monday for his criticism of Rep. John Lewis (D-Ga.).

"To demean John Lewis is to demean all those who give their lives for a cause bigger than themselves," Warren said at an Martin Luther King, Jr. Day event. "It is wrong, and we must stand up and say it is wrong."

"When Donald Trump says that our friend John Lewis is all talk and no action, he is talking about a man who was beaten to the ground with a club, his skull fractured, while leading a peaceful march for voting rights," she continued.

The president-elect sparked bipartisan backlash over his war of words with Lewis last last week. Trump accused Lewis of having a "crime infested" congressional district, after Lewis said the real estate mogul was not a "legitimate president."

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Democratic lawmakers, as well GOP Sen. Ben Sasse (Neb.), defended Lewis, noting his role as a key civil rights leader.

On Monday, Warren recounted visiting Selma, Ala., with Lewis, noting he told her that "'sometimes it is important to cause necessary trouble.'"

"Now, more than ever, it is time once again for all of us to cause 'necessary trouble.' All of us. Not just congressmen, not just senators - all of us. We all have an obligation to act," Warren added.

Trump responded to Lewis' criticism on Saturday evening, urging Lewis to work with him on "crime-infested inner cities" because he can "use all the help I can get."

Warren—who joined the Senate in 2013 and rose quickly through party ranks—is increasingly using her status as a liberal favorite to take on the incoming Trump administration.

She added on Monday that "there is no place for bigotry" and hit Trump over the hardline immigration stance he took during the presidential campaign.

"To build a future in this country, we cannot ignore the threats of the president elect to round up our fellow human beings and put millions of them out of this country, to tear millions of families apart," she said.

Trump has appeared to soften his initial pledge to deport 11 million undocumented immigrants, saying he will focus on those with a criminal record.

Warren was pressed after her speech Monday on whether she agrees with Lewis that Trump is an illegitimate president, but sidestepped the question.