DETROIT — Few things illustrate the fundamental shift of the Democratic Party as much as the debate stage lineup Tuesday night: Sens. Elizabeth Warren and Bernie Sanders splitting center stage — likely defending the sweeping progressive agendas they've popularized that are now literally at the center of the party's ideological struggle.

Though Sanders and Warren have been jockeying for top spots in recent polls — sometimes at the expense of the other — advisers inside and close to both campaigns have little, if any, expectation for drama ... at least between the two progressives.

"I do expect some fireworks," Adam Green, whose Progressive Change Campaign Committee has backed Warren, told NBC News, "but not between them."

He predicts instead that "Warren and Sanders will be a one-two punch ... mutually reinforcing each other's call for a bold, progressive, transformational presidency."

Advisers and strategists see a potential for the first night of the debate to turn into a referendum on Warren and Sanders' agenda on issues — like free public college and "Medicare for All," policies that others on the stage, like former Rep. John Delaney of Maryland and former Colorado Gov. John Hickenlooper, have railed against as "socialism."

Campaigning in New Hampshire Saturday, Warren said she's "ready to defend all of my plans" on stage Tuesday, while also channeling her longtime friendship with Sanders — "since long before I ever got into politics."

Sander's campaign manager, Faiz Shakir, also struck a similarly friendly note.

"There are other candidates on that stage, and also the next night, who believe in a middle ground approach, so I think like you're going to have Elizabeth Warren and Bernie Sanders end up agreeing on a lot of fundamental policy issues," Shakir told NBC.

For Sanders, Tuesday may be less about his competition with Warren for the party's progressive base and more about drawing a contrast against someone not on Tuesday night's debate stage: former Vice President Joe Biden, who will debate on Wednesday, the second night of the debate.

Sanders, I-Vt., will not be able to have the on-stage confrontation with Biden that his top advisers prepared him for before the first debate; however, senior advisers acknowledge a shift in strategy in the weeks since. That's meant an increased focus on "Medicare for All" on the campaign trail, leading to direct exchanges with Biden as he criticizes Sanders' signature proposal for being too disruptive.

Sanders' team wants that debate over what they call "bold and structural solutions versus gradual incrementalism" to drive the conversation on Tuesday night, despite the absence of the former vice president.

And it's one Warren, D-Mass., is ready for, too.

"I know what I'm going to be talking about," she said Saturday before launching into the elevator pitch for her candidacy. "A country that's working great for the wealthy and the well connected — just not working for anyone else. And we get a chance to change that in 2020."