Progressive frontrunners face off at CNN debate which may also offer opening for centrists

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.