HOUSTON — Elizabeth Warren has a Bernie Sanders problem — and she needs a plan to solve it, fast.

A race that began with Warren being "with Bernie" on progressive policies like "Medicare for All" has become one in which she is trying to make it clear to voters that she's an alternative to the senator from Vermont, who has assumed the front-runner position in the Democratic presidential field.

But while other candidates launch broadsides against Sanders — on both policy and personality — Warren has been more limited in her ability to attack, not only because of some overlap in their support but also because of mutual policy underpinnings to their campaigns.

"That's a box, in some ways, she has created for herself," Democratic strategist Joel Payne told NBC News. Warren "doesn't get the full credit from the left" for policy goals shared with Sanders, Payne said, but she does "get the same stigma" of being considered too radical by the middle of the Democratic Party.

That's not to say she isn't making the differences known, anyway. After a disappointing fifth-place finish in South Carolina on Saturday, Warren took aim at her top-tier rivals, specifically Sanders and Joe Biden.

"This crisis demands more than a former vice president so eager to cut deals with Mitch McConnell and the Republicans that he'll trade good ideas for bad ones," she said of Biden before moving on to Sanders, whom she described as "a senator who has good ideas but whose 30-year track record shows he consistently calls for things he fails to get done and consistently opposes things he nevertheless fails to stop."

It was about the most critical Warren has been of Sanders so far in the campaign, but the stakes are getting higher.

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Warren's campaign has sought to break her out of the so-called lanes that tend to dominate political coverage, pitching her as a coalition builder who can reach out to more moderate Democrats, while also hoping that she can marshal her progressive bona fides to coalesce the left. But Sanders has kept a strong hold on his supporters, and after a decisive top finish in Nevada, he has left the rest of the field jockeying to be the alternative candidate.

That's the role Warren now hopes to assume.

After middling finishes in Iowa and New Hampshire, Warren began sharpening her attacks on opponents and even showing new willingness to take on Sanders — leaning into contrast, as opposed to all-out attack, that hinges on her ability to actually enact their mutual agenda.

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Progressives like Rep. Ro Khanna, D-Calif., who is backing Sanders, said he had appreciated the tenor of the Charleston debate, at which Warren accepted the policy premise underlying both campaigns and instead simply argued that "I think I would make a better president than Bernie."

"The way that she's drawn the contrasts are within the bounds of respectful debate," Khanna said in an interview Friday before the senator's speech in Houston, conceding that Warren "did Bernie Sanders a huge favor in the last two debates by effectively putting a real dent in [Mike] Bloomberg's candidacy" during the debates.

To him, both the Warren and Sanders campaigns have always been inextricably linked. "They both fundamentally support a progressive policy," Khanna said, "and the movement is more important than the individual differences. And I think they both, to their credit, have largely taken that approach."

But contrast isn't just for onlookers spoiling for fireworks on a debate stage or on the airwaves. Warren's more explicit comments helped voters like Taylor Coleman make her choice — ultimately opting to back Warren over Sanders. At a Warren field office in Aiken on Friday, Coleman said: "I think that it was when she started to kind of pull herself away that I began to lean more towards her policy."

Still, she worries about whether the contrast was "a little late in the game."

The new strategy could also come at a price— one voiced by Julia DiTillio, a Warren supporter who went to canvass for her Friday in South Carolina.

"I think if she had come out swinging too early, she probably would have been accused of being shrill," DiTillio said. "She is going to have to take a more, not aggressive, but more of an offensive standpoint. I just think that she will need to do it without bringing the whole party down."

Regardless of any rhetoric or posturing, the two progressives are headed for a clash Tuesday in Warren's home state, Massachusetts, where Sanders rallied thousands of supporters Saturday and has taken a lead in recent polls.

Warren's team isn't promising a win in her home state or anywhere else — but in the immediate hours after South Carolina's results, it was promising something else: that it would be in this for the long haul, with both the will to stay in and a plan to do it.