Detroit (CNN) The second round of Democratic debates kicks off Tuesday night in Detroit with the primary's leading progressives, Sens. Bernie Sanders of Vermont and Elizabeth Warren of Massachusetts, sharing a stage with a handful of their most vocal moderate rivals.

Warren and Sanders are both running on the promise of a universal health care program run by the federal government ("Medicare for All") and a crackdown on powerful business interests, from Wall Street to the pharmaceutical giants. But they will be flanked at the Fox Theatre by a range of skeptics, from Mayor Pete Buttigieg of South Bend, Indiana, to Minnesota Sen. Amy Klobuchar and former Colorado Gov. John Hickenlooper

The debate could also be the last best chance for former Texas Rep. Beto O'Rourke to deliver on the early promise of his campaign, which has floundered in the polls and with donors. O'Rourke has signaled he will try to revive his bid with a more aggressive approach in Detroit as he tries to channel the excitement that drove his near-miss 2018 Senate campaign.

Here's what to watch for in Tuesday night's debate:

1. Toe-to-toe or side-by-side? Warren and Sanders share the stage

If it's debate night drama you're craving, both of these campaigns have a warning: look elsewhere

Sanders and Warren will be the highest-polling candidates onstage Tuesday. The pair run about even in a handful of early state surveys, and they share a similar message. While the demographic look of their support is different , it skews predictably left. If either is going to bring together a winning majority next year, they will need to build it up from a bedrock of dedicated progressive voters and activists.

If the past few months have been any indication, Medicare for All will come under attack by the likes of Hickenlooper, former Maryland Rep. John Delaney , Klobuchar and others. Buttigieg and O'Rourke, too, could seek to contrast their support for modified takes on the plan -- which allow some room for private insurers to operate -- with the progressive standard, which does not.

2. O'Rourke tries to rescue his chances

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O'Rourke roared into the campaign with a massive first quarter fundraising haul and the affection of Democrats around the country following his energetic and innovative, albeit unsuccessful, Senate campaign.

But after a few months on the national stage, the energy is running low and O'Rourke will need to turn it up to reinsert himself into the heat of the primary fight. That means a dialed up debate prep regimen and the promise of a more aggressive performance than the one he put on during the last debate in Miami.

How to accomplish that without risking his generally affable, optimistic brand will be a challenge -- and remains a mystery. He is ideologically to the left of the other low-polling candidates onstage, but to the right of Sanders and Warren, most notably on health care.

O'Rourke could choose to zero in on Buttigieg, with whom he's sparred a bit over the last few weeks. But his most intense recent rhetoric has focused on President Donald Trump. With Trump's continued racist attacks on members of Congress, the Texan could try to direct his sharpest barbs at the White House -- something no Democrat will hold against him.

3. The progressive platform on trial

Medicare for All, the Green New Deal, free college, student debt forgiveness, stricter financial regulation, new taxes on the wealthiest Americans, trade and US policy in the Middle East -- it will all come under intense scrutiny Tuesday night.

The issues that Sanders mainstreamed during his 2016 presidential run and Warren has spent years advocating for have galvanized the base but also created clear dividing lines within the party.

Tuesday night, with Sanders and Warren (literally) center stage, could turn into a referendum on the direction Democrats take going into their general election showdown with Trump.

The arguments are familiar by now. Moderates like Ohio Rep. Tim Ryan, Klobuchar, Hickenlooper and Delaney are warning that a turn left would allow Trump and the Republicans to label Democrats as socialists. Progressives like Warren and Sanders argue that embracing the grassroots and turning out new voters is the way to win. And Buttigieg and O'Rourke, whose views on the question seem to cut down the middle, are pushing for a more cautious move to the left.

The fireworks, then, could come when the moderate faction begins to throw darts at Sanders and Warren, trying to poke holes in their agenda and convince Democrats that their path is doomed. Those broadsides will be met with equal and opposite force -- embracing the fight is as core to the progressives' brand as the policies themselves.

It's a debate-within-the-debate both Warren and Sanders will be glad to take on. Given the state of the horse race, there will also likely be jockeying among the moderates to land the harshest, or most memorable, blow. Their risk: drowning each other out. Their opportunity: emerge as a moderate alternative to former Vice President Joe Biden.

4. The man who won't be there

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Biden figures to have his hands full on night two, but he will also be present -- as conjured up by his rivals -- on Tuesday, especially when it comes to health care.

The Sanders campaign has directed the lion's share of its attacks at the former vice president, who has been openly, and sometimes mockingly, skeptical of the party's move left and, specifically, the political prospects of Medicare for All. His objections and stated concerns have occasionally been misleading -- like when he suggested a transition to single-payer could leave people with gaps in coverage -- and quickly met with a series of stinging rebuttals from the Sanders team.

Warren has been less obviously in search of fronts for engagement with Biden, but their history is no secret . He remains the standard-bearer for a Democratic Party she argues is out of touch with working-class voters and too closely aligned with the financial industry. Whether she calls out Biden by name or keeps her message more general, Warren -- separated from him by a random draw for a second straight round -- could still use the big audience to cast herself as the progressive wing's best answer to his politics.

Biden's absence is less of a conundrum for the rest of the field, which is closer to him ideologically but far off his pace in the polls. For those candidates, the goal will be to offer up a similar policy agenda in what they hope is received as a more appealing -- or at least younger -- package.

5. Pete Buttigieg: Can he take the next step?

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Buttigieg's candidacy is at a crossroads. The South Bend mayor is printing campaign cash, but his embrace of big dollar donors to supplement his grassroots support makes it difficult to measure precisely where he stands.

Despite the strong fundraising numbers, he's mostly seen his polling flatten out. His campaign, though fundamentally strong so far out from Iowa caucuses, has yet to carve out a clear base of support.

Amid that uncertainty, Tuesday's debate could represent a turning point.

Buttigieg's politics don't fit neatly into the Democratic Party's progressive-moderate-centrist divisions. He has repeatedly rejected the most popular line of attack against Sanders, that the Vermont senator's democratic socialism would be used against the whole party. Buttigieg's rationale is that Republicans will lodge those charges against the nominee whomever it is, so the wiser path is, as he said recently in Iowa, to "just do what we think is right, make the case for it, and then let (the GOP) do what they want."

But he does not support Medicare for All and has rejected the idea of free college.

Buttigieg will have every opportunity to carve out his place on Tuesday to make his generational argument for change -- and why he's the one to deliver it. In a campaign that has begun to see some consolidation of support around Biden, Sanders, Warren and Harris, it's an opportunity he simply can't pass up.

Some of Buttigieg's most animating moments have come when he discusses his own faith and how it has guided his worldview. Those moments have distinguished him from the rest of the Democratic field.

6. Can Steve Bullock bust the governors' slump?

There will be one new face on the stage over these next 48 hours: Montana Gov. Steve Bullock.

Bullock's late entry into the race saw him closed out of the Miami debates, but he qualified for the Detroit stage and will, on Tuesday night, be given his introduction to a national audience of Democratic voters.

Even a cursory glance at his Twitter feed makes clear the message he's banking on to break through: that as the popular Democratic governor of a red state, he knows firsthand what it takes to operate effectively in a politically divided government.

Whether that record will be enough to stir up the Democratic primary voter base is another question. Hickenlooper, a popular former governor from another Western state, has so far failed to catch on and the crowded debate stage will make Bullock's task that much more difficult.

But all it takes, with so many months still before the first ballots are cast, is a moment -- that one viral clip or exchange that so many candidates are after, but few can will into reality. Bullock is no firebreather, further complicating the task, so expect him to lean heavily on his status in Montana and hope that viewers come away wanting to know more.