LAS VEGAS — Bernie Sanders "has a lot of questions to answer" about how his supporters attacked members of a union online after they criticized his "Medicare for All" health care plan, Elizabeth Warren said Monday.

"I've said before that we are all responsible for what our supporters do, and I think Bernie has a lot of questions to answer here," Warren, a Democratic senator from Massachusetts, said in an interview in which she offered rare criticism of her fellow progressive Sanders, an independent senator from Vermont.

"I am particularly worried about what happened in the attacks on members of the culinary union, particularly on the women in leadership," Warren said. "That is not how we build an inclusive Democratic Party. ... We do not build on a foundation of hate."

Members of Culinary Workers Union Local 226, which represents 60,000 workers in Nevada's gaming and hospitality industries, said this week that Sanders' supporters had "viciously attacked" its members after leadership warned about the risk to their negotiated health plans under a "Medicare for All" system.

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Sanders has said the attacks on union leadership were "not acceptable."

"I don't know who these so-called supporters are," he told PBS in an interview Thursday. "We're living in a strange world on the internet. And sometimes people attack people in somebody else's name. But let me be very clear. Anybody making personal attacks against anybody else in my name is not part of my movement."

While Warren isn't the only presidential candidate to criticize how Sanders has handled the so-called Bernie Bros online culture — former Vice President Joe Biden also urged Sanders to take accountability for "misogynistic" attacks — her comments mark a new willingness to draw stark contrasts with Sanders at a crucial point in the Democratic presidential race. Warren finished third in the Iowa caucuses and fourth in the New Hampshire primary, and she is looking to reinvigorate her campaign in the Nevada, South Carolina and Super Tuesday contests.

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That could begin with an appearance on the debate stage, the cycle's ninth gathering of candidates, this one hosted by NBC News and MSNBC. And after Sen. Amy Klobuchar of Minnesota boosted her fortunes with a strong debate performance in New Hampshire, the pressure is on for candidates to deliver.

But some Warren supporters wonder whether being more "aggressive" on the stage is a winning formula.

"I think women get a bad rap when they are aggressive," Sandy Laboy said Monday at a Warren campaign event.

Warren said: "This is what women face all the time. It's always too much of this or too much of that. But you put your head down, doing your job, and you keep on going. Or you might say: We persist."