Polls show that Massachusetts Sen. Elizabeth Warren is now a front-runner for the 2020 Democratic nomination. And on Tuesday night in Ohio, her 11 rivals acted like it.

The party's fourth presidential debate, hosted by CNN and The New York Times, showcased the shifting dynamics of the Democratic primary. Warren was under attack all night -- though it's far from clear whether anyone dented her status at the top of the field. Former Vice President Joe Biden, the other front-runner, stayed out of the fray for much of the event -- until a late clash with Vermont Sen. Bernie Sanders that featured the same one-on-one, progressive-against-moderate battle that defined the 2016 Democratic contest.

And moderate candidates -- some, like South Bend, Indiana, Mayor Pete Buttigieg, fighting to climb into the top tier; others, like Minnesota Sen. Amy Klobuchar, just desperate to make the next debate stage -- dropped the euphemisms and pressed their progressive foes in direct and sometimes personal terms.

Here are seven takeaways from Tuesday night's debate:

Warren under attack for the first time

Warren's months-long march to the front of the polls finally put her in the position of being the most heavily targeted and scrutinized candidate on stage.

Buttigieg and Klobuchar led the charge, assailing Warren over her answers on health care.

Bernie Sanders won the night

And it had nothing to do with what happened on stage.

In the debate's final moments, the Washington Post broke the news that New York Rep. Alexandria Ocasio-Cortez plans to endorse Sanders. CNN then reported that two other members of the "Squad," Minnesota Rep. Ilhan Omar and Michigan Rep. Rashida Tlaib, will do the same.

Echoes of 2016 as the front-runners fight

For most of the night, as Warren wore the biggest target, Biden slipped into the background. That changed near the end of the debate, when the field's top tier -- Biden, Warren and Sanders -- finally unloaded on each other.

The question that loomed over their: Biden has a lengthy record -- but is it one that's in line with where the Democratic electorate is now?

Trouble for Biden?

For the former vice president, fading into the background of a debate is a troubling sign because of what it suggests: that his foes view him as less of a threat than they once did.

But just as the good news had come late for Sanders, the real bad news for Biden's campaign came even later Wednesday night.

A more aggressive Buttigieg

The South Bend, Indiana, mayor had made it obvious he planned to come out swinging. In the days before the debate, he'd launched an ad that was critical of Warren and Sanders over Medicare for All, criticized Warren's grassroots fundraising strategy for the general election as being reliant on "pocket change," and attacked O'Rourke over his support for mandatory buy-backs of assault-style rifles in an interview on Snapchat's "Good Luck America."

The exchanges with Warren over health care might be the night's most memorable.

Klobuchar, unleashed

With the Democratic National Committee raising its fundraising and polling thresholds for the November debate, Klobuchar walked on stage facing the real possibility that this debate could be her last.

Her response: Go hard at the Democratic primary's most ascendant candidate, Warren.

Yang's 'Freedom Dividend' gets an airing

Andrew Yang launched his presidential campaign in 2017 with a plan to give every American $1,000 a month to combat job losses and automation -- and very little attention from media and voters.

Almost two years later, Yang's plan for a universal basic income, which he's calling a "freedom dividend," remains his signature policy proposal. But his impact on the race has increased dramatically -- a reality that was on display on Tuesday night when the candidates on stage debated a universal basic income and job losses to automation in depth on national television.

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