It was Round Four of the Democratic debates on Tuesday, with 12 candidates squaring off in Westerville, Ohio.

What were the key takeaways from the night?

Warren in the firing line

Sen. Elizabeth Warren Elizabeth WarrenOvernight Defense: Appeals court revives House lawsuit against military funding for border wall | Dems push for limits on transferring military gear to police | Lawmakers ask for IG probe into Pentagon's use of COVID-19 funds On The Money: Half of states deplete funds for Trump's 0 unemployment expansion | EU appealing ruling in Apple tax case | House Democrats include more aid for airlines in coronavirus package Warren, Khanna request IG investigation into Pentagon's use of coronavirus funds MORE (D-Mass.) has jumped to the top of some national polls recently, and her status as the new front-runner was underlined when several of her rivals attacked her.

They clearly see an urgent need to curb her momentum.

ADVERTISEMENT

South Bend, Ind., Mayor Pete Buttigieg Pete ButtigiegBillionaire who donated to Trump in 2016 donates to Biden The Hill's Morning Report - Sponsored by Facebook - GOP closes ranks to fill SCOTUS vacancy by November Buttigieg stands in as Pence for Harris's debate practice MORE (D) and Sen. Amy Klobuchar Amy Klobuchar3 reasons why Biden is misreading the politics of court packing Social media platforms put muscle into National Voter Registration Day Battle lines drawn on precedent in Supreme Court fight MORE (D-Minn.) were among the most aggressive in jabbing at Warren early on, though others, including Sen. Kamala Harris Kamala HarrisTexas Democratic official urges Biden to visit state: 'I thought he had his own plane' The Hill's Campaign Report: Biden on Trump: 'He'll leave' l GOP laywers brush off Trump's election remarks l Obama's endorsements A game theorist's advice to President Trump on filling the Supreme Court seat MORE (D-Calif.) and former Rep. Beto O’Rourke (D-Texas), joined the fray later.

Warren by no means crumbled. But she was put on the defensive as she has never been before.

On her "Medicare for All" plan, Buttigieg argued it limited choice, while Klobuchar hit her refusal to acknowledge that paying for it would almost certainly involve tax hikes.

Warren’s argument on the tax issue — that voters are concerned about overall costs, and that any tax increase would be offset because there would be no private insurance premiums — is logically defensible. But her rivals will exploit her tendency to sound evasive on the tax point.

The Massachusetts senator seemed to become more comfortable as the night wore on. And her basic position that Democrats need a platform more ambitious than one that “nibbles around the edges of the big problems in this country” has real magnetism for progressive voters.

Warren survived her first turn in the firing line competently. But everyone will be watching the polls in coming days to see whether the sustained attacks on Tuesday slowed her march.

A bad night for Biden — again

Indifferent debate performances have been a problem for former Vice President Joe Biden Joe BidenFormer Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick Bloomberg rolls out M ad buy to boost Biden in Florida MORE before — and he failed to break the streak on Tuesday.

The 76-year-old faded into the background for large swathes in Westerville. When he did gain the spotlight, his answers were prone, once again, to lack sharpness or punch.

Even though Biden is two years younger than Sen. Bernie Sanders Bernie SandersSirota reacts to report of harassment, doxing by Harris supporters Republicans not immune to the malady that hobbled Democrats The Hill's Morning Report - Sponsored by Facebook - Republicans lawmakers rebuke Trump on election MORE (I-Vt.), he continues to sprinkle his remarks with much more anachronistic phrases. On Tuesday, he talked about people “clipping coupons in the stock market” — a reference that seemed several decades out of date.

Biden’s backers would argue that his support has always come from older, more centrist Democrats and that he is the strongest candidate to take on Trump.

But there was nothing remotely dominant about his performance on Tuesday night.

Bernie bounces back — with AOC’s help

Sanders came into this debate facing serious questions about his health. It was the 78-year-old’s first major appearance since having a heart attack.

The Vermont Independent began putting those worries to rest with a typically feisty performance. There was no sign of a lack of vigor or stamina.

But the biggest boost to the veteran democratic socialist came from outside the debate hall.

As the clashes in Westerville were winding down, The Washington Post broke the news that he would be endorsed by Rep. Alexandria Ocasio-Cortez Alexandria Ocasio-CortezOn The Money: Anxious Democrats push for vote on COVID-19 aid | Pelosi, Mnuchin ready to restart talks | Weekly jobless claims increase | Senate treads close to shutdown deadline McCarthy says there will be a peaceful transition if Biden wins Anxious Democrats amp up pressure for vote on COVID-19 aid MORE (D-N.Y.) at a Saturday rally in Queens, N.Y. Soon after, news emerged that Reps. Ilhan Omar Ilhan OmarOmar urges Democrats to focus on nonvoters over 'disaffected Trump voters' Omar fires back at Trump over rally remarks: 'This is my country' Trump attacks Omar for criticizing US: 'How did you do where you came from?' MORE (D-Minn.) and Rashida Tlaib Rashida Harbi TlaibTrump attacks Omar for criticizing US: 'How did you do where you came from?' George Conway: 'Trump is like a practical joke that got out of hand' Pelosi endorses Kennedy in Massachusetts Senate primary challenge MORE (D-Mich.) would also back him.

The endorsement from Ocasio-Cortez was arguably bigger news than anything that happened on the debate stage on Tuesday. The young left-wing icon’s endorsement may not be a surprise in itself — she worked as a volunteer organizer for Sanders’s 2016 campaign — but the timing delivers a jolt of excitement to his campaign at just the right moment.

It will also come as a disappointment to Warren, whom Ocasio-Cortez had also previously praised.

Buttigieg seizes his moment

Buttigieg was the single standout performer on Tuesday. He took the fight to Warren, was prominent in the key early stages of the debate and made his case more broadly as a candidate able to connect with voters beyond the liberal base.

Buttigieg has always been an effective television performer, and he has showed startling fundraising strength.

He has, however, struggled to translate those assets into real momentum in the polls.

The 37-year-old mayor’s path into serious contention lies in the possibility that he could supplant Biden as the centrist standard-bearer against the progressive wing of the party, represented by Warren and Sanders.

He helped himself in a big way on Tuesday — and Biden’s weakness also played into his hands.

Harris misfires with Warren attack

Harris shone in the first Democratic debates in Miami in late June, but she has faded in the polls since then.

She enjoyed a good moment early on Tuesday when she turned a general discussion of health care to the topic of women’s reproductive rights — a winning move with the crowd in the hall, and presumably with a lot of Democratic voters.

Just as memorable — but for the wrong reasons, from Harris’s perspective — was a later exchange with Warren on the subject of President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE’s Twitter account.

Harris pressed Warren on why she wouldn't support Harris’s push to have Trump banned from the social media platform. But Warren easily turned the attack around by saying that she was focused on removing Trump from the White House, not just from Twitter.

Harris persisted with the confrontation but the overall effect was to diminish herself rather than Warren, with the California senator seeming mean-spirited and somewhat petty.

It was a bad moment for a candidate who could ill afford one.