(CNN) Elizabeth Warren's shock-and-awe attacks on Michael Bloomberg have won her the hearts -- and dollars -- of many liberal Democrats, infusing new oxygen into a campaign that looked to be on the verge of collapse after two disappointing finishes in Iowa and New Hampshire .

But the Massachusetts senator has also been increasingly pointed in her criticism of the frontrunner and longtime ally, Sen. Bernie Sanders . Warren's more cautious tone when it comes to Sanders underscores the delicate balancing act she faces in seeking to raise questions about his prospects while, at the same time, attempting to win over some of his progressive backers.

That dynamic was on display during the debate on Wednesday night, when Warren inserted a jab at Sanders on health care, arguing -- in what has become a theme in her remarks about him -- that while she broadly agrees with the Vermont senator on policy, the tone of his campaign is a turn-off to all but the true believers.

"Bernie has a good start, but instead of expanding and bringing in more people to help, instead, his campaign relentlessly attacks everyone who asks a question or tries to fill in details about how to actually make this work," Warren said. "And then his own advisers say, yeah, probably won't happen anyway."

One night later, at a CNN town hall in Las Vegas, Warren questioned how any candidate -- "including Sen. Sanders" -- planned to pass their agenda through a divided Congress without calling for an end to the Senate filibuster. And though she did not go as far as former South Bend, Indiana, Mayor Pete Buttigieg, who recently said he worried Sanders would be a general election liability, Warren also cast doubt on his prospects in November.

Democratic voters "are nervous about a narrow vision that just doesn't speak to the Americans who see real problems and want to see real change," Warren said, addressing her centrist rivals, before turning to Sanders. "But they are also worried about gambling on a revolution that won't bring along a majority of this country."

"I am a Democrat, through and through," she said. "I speak to all parts of our party and with things like the wealth tax, speak to independents and Republicans."

Shortly after, during an interview on MSNBC, Warren was asked how she and Sanders differed. It is a question that's been posed to her dozens of times over the past 13 months. For most of that time, she has avoided any kind of answer that might get reporters' pens out. But on Thursday night, Warren swung the ax.

"I get real stuff done. I have rock solid values and I get stuff done. I get hard stuff done," she said, pointing to her lead role in creating the Consumer Financial Protection Bureau during the Obama administration. "I don't want to be president just to yell at people, I want to be president to change things."

Earlier in the day, Warren also engaged in unusually direct terms on a question about Sanders' health records. He has released three doctors' letters since his October heart attack, but despite initially promising more information, has recently said his campaign has no plans to make any further disclosures.

"He had made a promise to release all his medical records and I thought that was what he was going to do," Warren said. Pressed on whether she believed Sanders had lived up to that promise, Warren was blunt.

"I don't think that's a question of opinion," she said of the letters. "Those aren't medical records."

The swipes have largely passed without notice as the Democratic field closes ranks in opposition to Bloomberg, who arrived late in the process and has spent more than $400 million while sitting out the primary's first four contests. Back in January, progressive groups panicked and sought to broker a peace between Warren and Sanders after the two clashed over the contents of a private conversation from 2018 , in which Warren said Sanders told her he did not believe a woman could win the presidency.

This time around, though, the state of the race is different.

Sanders is leading the pack on the eve of the Nevada caucuses and Warren is fighting to recapture the excitement that animated her campaign late last summer and into the fall. Since then, Warren has sought to make the case, in often stark terms, that she is the only Democratic candidate who can being together a fractured and factional party before November.

Warren's quiet offensive has not elicited much of a response from the Sanders campaign, which declined to comment for this story. Sanders supporters online, whose aggressive tactics have been drawing headlines almost daily, have also been relatively muted in their response, instead focusing their ire on Bloomberg.