But they don't want a prize fight to determine which one will come out on top -- not now and, ideally, not ever.

As Warren and Sanders prepare now to take the debate stage in Detroit on Tuesday night, both candidates face pressure from the movement they helped build to stay focused on a bigger task: making the case for progressive governance and beating back the party's moderate wing, led by former Vice President Joe Biden. Sanders and Warren have mostly adhered to a mutual non-aggression strategy throughout the primary. Their campaigns are careful in how they talk about one another -- with top officials loathe to engage questions that would highlight potential differences.

When Sanders was asked last weekend in Iowa what he expected from sharing a debate stage with Warren, the Vermont senator said, "Intelligence" -- an answer that met with applause in progressive social media circles.

"That's what Bernie Sanders thinks," his chief of staff, Ari Rabin-Havt, told CNN days later. "He wasn't saying something cute. He was saying something that he truly believes about somebody who he is running against, but who he has immense respect for."

The Warren campaign has been similarly complimentary. In an email Thursday, communications director Kristen Orthman offered her own kind words, before returning to Warren's core message.

"Elizabeth considers Bernie a close friend and looking forward to sharing the stage with him and others," Orthman said. "Our strategy isn't a secret. It's always been to continue talking about Elizabeth's plans for big, structural change, and the grassroots movement that will make them happen."

Sanders and Warren share more than their progressive visions. They have also chosen top aides -- Sanders' campaign manager Faiz Shakir the highest-ranking among them -- who share a common line on their resumes: past positions in former Democratic Senate Majority Leader Harry Reid's office. Like Orthman, Rabin-Havt worked for the Nevadan, and he too was quick to pivot away from chatter over the prospect of a debate night clash.

"We view these debates as opportunities to share in a high-profile setting with the American people what Bernie Sanders believes and how he will govern as president regardless of who he is on stage with," Rabin-Havt said. News outlets and political pundits, he added, "want to try to create the idea of an Elizabeth Warren-Bernie Sanders battle that doesn't exist."

Another top Sanders campaign official reiterated that point, telling CNN that the senator had no plan to go out of his way to draw distinctions with Warren. The official said Sanders wouldn't shy away from a direct question on the subject, but suggested their differences were so minor that a dramatic debate night flashpoint seemed unlikely.

Warren in the first debate made a point of pulling Sanders close. She raised her hand to signal her support for eliminating private insurance as part of implementing "Medicare for All" and, when the follow-up came, she declared, "I'm with Bernie" on health care.

Progressive leaders are insistent that any other approach would be counterproductive. At this stage in the primary, they told CNN, the campaigns should remain focused on sharpening their already stark disputes with moderate Democrats.

"They're going to be the two progressives on stage with all moderates. So if I were Bernie Sanders and Elizabeth Warren, my main priority would be (to define) how we continue to build the movement in order to defeat Donald Trump in 2020," Justice Democrats executive director Alexandra Rojas said. "I think it would be a distraction to (focus on) anything other than that we need big solutions to some of the biggest problems that we have."

Maurice Mitchell, national director of the Working Families Party, said Warren and Sanders had more to gain by uniting against Democratic opponents of policies like Medicare for All and free college than in dissecting how they split or differ.

"Honestly, either one of them would have more to lose by engaging in a back-and-forth and needling each other around important, but minor differences, compared to what's actually at stake, and the stark differences between them and some of the much more corporate actors, and them and Trump," Mitchell said.

Sanders and Warren have built very different coalitions through the opening months of the primary. The Vermont independent's support, according to recent polling, comes from younger, working-class and less-educated voters. His colleague from Massachusetts has done better with older, college-educated voters, and women. But progressive activists aren't focused on simply uniting those cohorts -- they want to grow them first.

"Progressive voters, primary voters are going to be really excited to have two champions on one stage, who will be doubling up on making that case," said Indivisible's national political director, María Urbina, another Reid alum. "And I think that the distinctions that you're going to see are certainly going to come more from other folks on that stage."

In The Intercept , author and climate activist Naomi Klein delivered a more direct message last month to the increasingly influential site's readers: "Forget Bernie vs. Warren. Focus on Growing the Progressive Base and Defeating Biden."

"The very last thing we need is for the two strongest left/progressive candidates and their supporters to tear each other apart for the next eight or so months, in a desperate bid to discredit a perceived rival," Klein wrote. "What should be happening instead is exactly what Sanders and Warren have been doing (with only a couple minor lapses): steadily building their bases by talking about ideas and strategies, thereby sharpening the contrast — in policies, track record, and electability — with Biden."

Rojas offered a similar plea.

"I want Bernie and Warren to go on the offensive and push our progressive agenda where the Amy Klobuchars, the John Delaneys, are going to operate from a place of fear -- fear of what the Republicans are going to say," she said.

If sparks do fly on Tuesday night, that seems to be the most likely source. Delaney has been among Sanders' harshest critics. In late June, the former Maryland congressman's campaign warned that Sanders, with his hardline support of Medicare for All, risked hobbling all Democrats heading into 2020.

"If Senator Sanders wants to run as a Socialist, he shouldn't use the Democratic Party as his vessel to reelect Donald Trump," a spokesman said.

Rabin-Havt said Sanders was focused on making his own case, but was prepared to defend his politics if confronted by the moderates onstage.

"They are more than welcome to yell that across the stage," he said. "If that's what they want to run on and believe is the most effective case to make before the Democratic Party, to get the nomination -- we've seen when (former Colorado Gov. John) Hickenlooper made that case in California, he was roundly booed. So they can make that case. It doesn't seem to be a very effective one."