Those candidates, including Minnesota Senator Amy Klobuchar, former Colorado governor John Hickenlooper, and Montana Governor Steve Bullock, could try to push out of the lower tiers of the large pack by launching themselves at Warren, or — more likely — Sanders.

But despite signs of tension between the campaigns in recent months, the debate is unlikely to feature a clash between the two for the mantle of the left. Instead, the self-proclaimed longtime friends may be too busy warding off attacks from five more moderate candidates they’ll also be sharing the stage with — all of whom are desperate for a breakout moment.

DETROIT — For the first time since the Democratic presidential campaign began, the race’s two leading liberal candidates — Senators Bernie Sanders and Elizabeth Warren — will appear on the same debate stage here Tuesday night.


Their goal is to avoid being cut from September’s debate because of low support in the polls or donations. With little to lose, those candidates could attack the two New Englanders for pushing liberal proposals that, they argue, could hurt the party’s chances in 2020.

“This debate is really important to all the candidates who are at risk of not being back on the debate stage and facing a sort of debate death sentence,” said Joe Trippi, a Democratic strategist who managed Howard Dean’s 2004 presidential campaign. “That’s why I think it’s going to be a much more aggressive debate than the first one.”

Instead of trying to subtly grab the title of most progressive candidate, Warren and Sanders may find themselves defending each other from criticism of their support for a Medicare for All health insurance plan, cancellation of student debt, and imposing higher taxes on the wealthy to pay for new programs.

Adam Green, co-founder of the Progressive Change Campaign Committee, which backs Warren, said he expects “a tag team act” against the moderates.


“It’s mutually reinforcing for Sanders and Warren to make the case for bold progressive transformational policy — it makes them each look better,” Green said.

This shared goal may be enough to overcome signs of tensions between the Sanders and Warren campaigns that have recently surfaced. Unnamed Sanders advisers questioned Warren’s electability and past claims to Native American heritage in news reports. Sanders himself suggested in a tweet last month that he — not Warren — is the “real threat to the billionaire class.”

Sanders quickly appeared on CNN to clarify that he was not targeting Warren, but he has reason to be wary of her. Sanders has seen his standing in early-state polls fall as Warren’s creeps up and even surpasses him in some surveys. And the two share similar messages, sometimes even appearing to race each other to release their latest policy proposals, as Warren did with a plan to cancel student debt two months before Sanders unveiled his.

But Sanders’ allies point out he does not appear to be personally behind the anonymous sniping from his campaign directed toward Warren, whom he considers a friend.

Jim Zogby, a board member of the Sanders-allied group Our Revolution, said he doesn’t expect the Vermont senator will go into the debate looking to target any of Warren’s weak spots. “They’re friends and he’s displayed no animus whatsoever and I don’t expect to see any on stage,” Zogby said.


Asked recently by reporters in Iowa what people should expect from him and Warren sharing a stage, Sanders responded, “Intelligence.”

Still, the two liberals have plenty of weak spots to press on each other, which could happen if the debate’s moderators ask pointedly about their differences. Warren could argue that she is a better standard-bearer for the party, since she is a Democrat while Sanders is an independent who identifies as a democratic socialist. And Sanders could make the case that he has been fighting for progressive values longer than Warren has.

But the Democratic race appears to be overdue for a moderate rebellion, not lefty infighting.

In last month’s debates in Miami, few of the more moderate candidates in the sprawling 2020 field took direct shots at their more liberal rivals, with fireworks flying mostly between Senator Kamala Harris of California and former vice president Joe Biden on race, and former secretary of housing Julian Castro and former representative Beto O’Rourke on immigration.

Aside from Biden, who is a front-runner in polls and will appear in Wednesday’s debate, more moderate candidates have largely failed to gain traction in surveys and media attention, even as party insiders fret that the 2020 race is revolving around what they see as proposals like Medicare for All and decriminalizing border crossings that are too liberal for average voters.

“If you win the nomination in a way that forecloses a path to victory in the general election, we will lose, and your name will go down in infamy,” former Chicago mayor Rahm Emanuel, a onetime Obama chief of staff wrote in a scathing op-ed this week about the Miami debates.


If a moderate candidate can make the case that he or she is more electable in 2020 by taking on Sanders or Warren, that could lead to a polling bump to help him or her appear in the next debate.

“One of them I think is going to emerge out of the debate as having a little bit of momentum,” Trippi predicted of the moderates.

In the first debates in Miami, neither Sanders nor Warren went on the attack against their rivals, and only Sanders faced any direct criticism of his policies. “If we don’t clearly define that we are not socialists, the Republicans are going to come at us every way they can and call us socialist,” Hickenlooper said, while Colorado Senator Michael Bennet pointed out Sanders’ home state of Vermont was unable to adopt a so-called single-payer health care plan similar to Medicare for All.

Sanders defended himself, pointing out how well he performs in some head-to-head polls against President Trump, and the exchanges were quickly overshadowed by the back-and-forth between Harris and Biden on busing to desegregate schools.

Sanders’ allies say he’s ready for more attacks along the same lines from Hickenlooper and others.

“There’s going to be people saying, ‘I’m going to score points by attacking Bernie,’” Zogby predicted. “There’s a perception that Bernie is vulnerable.”


Sanders, the field’s oldest candidate at 77, could also face a generational challenge, as Pete Buttigieg — the race’s youngest candidate, at 37 — will be standing right next to him.

If Sanders ends up as the debate’s lightning rod, the result may be a relatively quiet night for Warren, who has maintained her position in second or third in the polls since rising in the spring on the strength of a steady stream of progressive policy proposals.

“I don’t think the burden is on Elizabeth Warren here; I think she can keep her head down and get through it,” said Rebecca Katz, a Democratic political consultant in New York. “Since her launch, she very much has been the tortoise: slow and steady wins the race. She doesn’t have to have a breakout moment.”

Such a moment for Warren is far more likely when she eventually shares a stage with Biden, who has maintained his commanding lead over the field despite a shaky performance in Miami. Warren has already highlighted her differences in opinion with the former vice president on bankruptcy, while Sanders has poked at Biden’s fund-raisers targeting the wealthy and his more incremental policy approach.

“Everyone wants a piece of Biden,” said Brian Fallon, a former top Hillary Clinton aide and the founder of the liberal advocacy group Demand Justice.

Liz Goodwin can be reached at elizabeth.goodwin@globe.com. Follow her on Twitter @lizcgoodwin