(CNN) CNN's first Democratic debate on Tuesday night will showcase a match-up between Sens. Bernie Sanders and Elizabeth Warren, two icons of the progressive movement who are ultimately on a collision course for the party's presidential nomination.

Their encounter will kick off two televised clashes between Democratic White House hopefuls on successive nights that could significantly reshape the primary campaign and will, for the first time, test how party hopefuls handle Donald Trump's ugly campaign strategy rooted in direct racism.

The debates, each featuring 10 candidates, mark the biggest moment of the campaign yet and will further inflame plot lines stoked in June's frenetic first candidate free-for-alls.

Sanders and Warren were kept apart back then -- a quirk of the draw that gave Warren an uninterrupted chance to tout her progressive credentials that included a strong answer on her plans to promote greater economic fairness right out of the gate.

They are unlikely to replicate the stunning personal confrontation between Democratic front-runner former Vice President Joe Biden and California Sen. Kamala Harris that stole the spotlight in the first round of debates and could reignite in Wednesday night's second CNN showdown.

Sanders, from Vermont, and Warren, from Massachusetts, have mostly avoided direct confrontations in the campaign so far. And it may be that both have more incentive at this early stage of the race to draw distinctions with more centrist Democrats than to seek to pick holes in each other's campaigns.

But the comparisons between them will be unavoidable on Tuesday night and the meeting will sow the seeds for future disputes when direct combat between the pair becomes unavoidable.

They are often aligned on policy but Warren and Sanders do differ in tone. The Massachusetts senator has rejected the label "democratic socialist" that her Vermont colleague has embraced and that Trump has seized upon to brand the Democratic Party as extreme.

"Bernie has to speak to what democratic socialism is," Warren said in March.

"All I can tell you is what I believe. And that is there is an enormous amount to be gained from markets," she said, though qualified her remark by noting that markets need to have rules.

The presence on stage of Sanders and Warren on Tuesday could also provide an opportunity for more moderate candidates like Montana Gov. Steve Bullock -- making his debut on the Democratic debate stage -- or Minnesota Sen. Amy Klobuchar to argue that the party is drifting too far to the left on issues like health care and paying for college.

For now, the moderate lane looks blocked with Biden -- who sometimes sounds like he's running for Barack Obama's third term -- as the current front-runner. But it could open up if he stumbles again.

And the line-up also offers 37-year-old South Bend, Indiana, Mayor Pete Buttigieg, the chance to create a strong generational contrast with his older rivals as he seeks to broaden support that has brought him bumper fundraising figures but has failed to generate equally impressive polling numbers

The location of the debates -- in the battleground state of Michigan -- is a reminder of the task the eventual Democratic nominee will face in November 2020. The once-blue bastion fell to Trump in 2016 and is one of the Midwestern battlegrounds where Democrats must win back working-class votes to claim the White House.

Biden is under pressure on Wednesday to improve upon a halting first debate that highlighted some of the 76-year-old's political liabilities.

Sen. Kamala Harris, who made her mark last month by skewering Biden on issues of race, faces heightened expectations in the rematch. And Harris enters the second debate facing fresh criticism from her rivals that she is taking both sides on the question of a proposed new health care system, an ideologically defining question for Democrats.

But before Biden and Harris clash again, the fight for the party's Democratic Party's grassroots will be in the spotlight on Tuesday when Sen. Elizabeth Warren comes face-to-face with Sen. Bernie Sanders, who has done more than anyone to reinvigorate the left but now risks being eclipsed by the Massachusetts senator.

The two progressive icons have mostly avoided direct confrontations in the campaign so far. And it may be that both have more incentive at this early stage of the race to draw distinctions with more centrist Democrats they both see as too friendly to corporations than to seek to pick holes in each others campaigns.

While they are often aligned on policy, Warren and Sanders do differ in tone. The Massachusetts senator has rejected the label "democratic socialist" that her Vermont colleague has embraced and has been seized upon by Trump to brand her party as extreme.

"Bernie has to speak to what democratic socialism is," Warren said in March.

"All I can tell you is what I believe. And that is there is an enormous amount to be gained from markets," she said, though qualified her remark by noting that markets need to have rules.

Tuesday night's debate may also offer an opening for a centrist candidate, like Montana Gov. Steve Bullock -- making his debut on the Democratic stage -- or Minnesota Sen. Amy Klobuchar, to define themselves against the ultra-progressives. For now, the moderate lane looks blocked with Biden -- who sometimes sounds like he's running for Barack Obama's third term -- as the current front-runner. But it could open up if he stumbles again.

The first voting in the Democratic nominating race is more than six months away. But because the qualifying criteria for the next debate in September gets twice as hard, outsider candidates face a make-or-break moment and have every incentive to prey on top-tier candidates to create a viral television moment.

This week's contests are also the first time Democratic voters will see their candidates together since Trump's attacks on minority Democratic lawmakers tipped his 2020 hand.

The President has shown he is ready to enlist racist themes to fire up his base, a factor that will influence a Democratic contest featuring racially diverse candidates.

Trump, his tweeting thumbs at the ready, will likely pounce if the Democratic candidates embrace "The Squad" -- four female minority lawmakers he racially attacked when he said they should "go back" home -- even though all four are US citizens.

He has made clear he plans to use the four women of color, who he accuses of harboring anti-American sentiments, as an emblem of the entire Democratic Party.

Strong calls for more gun law reform from Democratic candidates following America's latest mass shooting -- in California on Sunday night -- are sure to come up in the debate.

But gun control, like Democratic pledges to reverse Trump's hardline border strategies, is an issue that can offer opportunities for the President as well as his foes.

Biden v Harris

The most eagerly awaited clash of the pair of prime time debates is the rematch between Biden and Harris.

The former vice president looked shocked and unprepared when Harris challenged him on his attitude toward opposition to government-mandated busing in the 1970s to integrate schools.

Biden's weak response to Harris' attack played into concerns among some voters that his age is a liability. It also threatened to undermine his claim that he would be the best Democrat to take on Trump -- a ferocious debater.

Any candidate can have a bad debate. But Biden surely can't afford another one.

The ex-vice president also needs to look out for Sen. Cory Booker. Like Harris, the New Jersey senator has attacked Biden's past stances on racial issues and criminal justice. And Booker is desperate for his campaign to catch fire. A fracas with Biden might provide a spark.

Brett O'Donnell, a former debate coach on Republican campaigns, said Biden needed to make tactical adjustments.

"I think Joe Biden's got to become an effective counter-puncher. You know, when you're the frontrunner, you don't want to initiate contact," O'Donnell told CNN's Poppy Harlow.

"The principle is, do no harm. At the same time, you don't want to appear so passive, as he did in the last debate."

Biden's aides have told CNN that the former Delaware senator will take a more aggressive approach on Wednesday.

"He isn't going to allow his record to be weaponized," an official said. "He isn't going to take hits from any of the candidates sitting down."

Still, the initial panic among some Democrats about the frontrunner after the first debate seems to have subsided.

A new Quinnipiac University poll Monday showed Biden backed by 34% of Democrats and independents who lean Democratic. Warren was on 15%, Harris had 12% and Sanders had 11%. Harris slipped 8% from a Quinnipiac poll taken shortly after her first debate showdown with Biden.

So far, Harris has seemed less assured in spontaneous situations than in implementing pre-cooked strategies in a debate. She has appeared for instance to reverse herself several times on a key issue in the Democratic race -- how to improve the US health care system.

On Monday, Harris released a 'Medicare for All' plan that preserves a role for private insurance in an apparent attempt to straddle the left and centrist wings of the Democratic Party.

The fight for the progressive left

Four years ago, Sanders dominated progressive territory in the Democratic presidential race in his ultimately unsuccessful bid to beat former Secretary of State Hillary Clinton.

This time, he's finding things a lot more crowded.

Sanders and Warren are closely matched in recent early state and national polls, suggesting that they are currently splitting the support of the most progressive activists.

The danger now for Sanders is that Warren, with her multiple plans and broader appeal, ends up pushing ahead of the Vermont senator and inherits the movement he did so much to build.

So he could come out swinging on Tuesday in an attempt to re-establish himself as the most powerful progressive.

Warren knows that dominating the left will not be enough -- after all it failed to secure victory for Sanders in 2016.

Her performance will be watched to see if she can make inroads with more moderate voters and African Americans -- two other crucial blocs of the Democratic coalition.

Given the presence of Warren and Sanders, Tuesday's first debate could also lay bare the ideological divide in the Democratic Party. Bullock and former Colorado Gov. John Hickenlooper have been critical of the party's evolution and could make a play for the center ground.

"I think as Democrats, we have got to be clear that we're not socialists," Hickenlooper told "CNN Tonight" earlier this month.

"I know we're not, but these large, expansive solutions to some of the vexing problems of America push people away in many cases," Hickenlooper said.

"To say that we're going to in four years take away all private insurance, ask 180 million Americans to give up their private insurance -- I don't think that's realistic and I think that's not how you win elections."

Other outsider candidates need big nights to keep their campaigns alive.

Former Texas Rep. Beto O'Rourke -- who ran Republican Sen. Ted Cruz close in the state's 2018 Senate race -- needs to restore his luster. Former Housing and Urban Development Secretary Julian Castro was strong in his first debate -- but has not seen a hoped-for polling bounce.

Other candidates like New York Sen. Kirsten Gillibrand, Colorado Sen. Michael Bennet, New York Mayor Bill de Blasio and Washington Gov. Jay Inslee know that time is running out for them to burst into the top tier of candidates.

Others, like businessman Andrew Yang and self-help guru Marianne Williamson, face a struggle to get equal time.