Elizabeth Warren took heat in the fourth Democratic debate. Here are the night's top moments

Sen. Elizabeth Warren has reached the top of the Democratic pack, and now she has the scars to prove it.

The Massachusetts senator weathered attacks throughout the fourth Democratic debate, which was held Tuesday night in Ohio. It was the first such event since Warren started beating former Vice President Joe Biden in several national polls.

Democrats also focused on the economy and the impeachment inquiry against President Donald Trump in a wide-ranging debate that, with 12 contenders on stage, was the most crowded in presidential history.

In addition to fending off the most attacks — 16, according to a tally by NBC News — Warren also spoke the most, clocking in at about 23 minutes. Biden spoke for about 16 minutes.

Here are Tuesday's top moments.

With no opening to statements to trudge through, the debaters wasted little time going after their main target, President Donald Trump. Their second target: Warren, whom they tore into after she repeatedly dodged questions about middle-class tax hikes under her health-care plan.

Warren, whose campaign has rocketed to front-runner status in recent weeks, supports the "Medicare for All" plan pushed by Vermont Sen. Bernie Sanders, a democratic socialist. She promises that the plan would raise taxes on the wealthiest Americans and lower costs for middle-class families. But she refused to provide a straight answer on whether middle-class taxes would go up.

"At least Bernie's being honest here," Minnesota Sen. Amy Klobuchar said of Sanders, who acknowledges a tax hike under the plan, which would abolish private insurance.

"I don't understand why you believe the only way to deliver coverage for everybody is to obliterate private plans," South Bend, Indiana, Mayor Pete Buttigieg told Warren.

Biden addressed Trump's allegations of nepotism and corruption related to work by the former vice president's son Hunter in Ukraine and China. The allegations have threatened to distract from Biden's candidacy.

"My son did nothing wrong. I did nothing wrong. I carried out the policy of the United States government in rooting out corruption in Ukraine," Biden said when pressed by the debate moderators. And Biden also went after Trump and his personal lawyer, Rudy Giuliani, who has been pursuing the claims against Biden and his family.

"The president and his thugs are flat lying. He doesn't want me to be the candidate. If I'm the candidate, he knows I will beat him like a drum," Biden said.

Hunter Biden has come under attack by Trump and his conservative allies over his work overseas while his father was vice president. Hunter Biden held a lucrative role on the board of a Ukrainian energy company, Burisma Holdings, while Biden was the point person for U.S. policy in Ukraine, and recently said he would leave the board of a Chinese private equity firm.

In an ABC interview that aired Tuesday, Hunter Biden defended himself and said he "did nothing wrong at all," though he conceded "poor judgment" for taking roles that complicated his father's presidential bid. And he chastised Trump and his supporters for spreading conspiracy theories related to his work.

"I gave a hook to some very unethical people to act in illegal ways to try to do some harm to my father. That's where I made the mistake," he said.

At the debate, Joe Biden repeatedly said his son's statement speaks for itself.

"My son made a judgment. I'm proud of what he had to say," Biden said.

In a heated faceoff, Buttigieg told former Rep. Beto O'Rourke that the problem with gun violence "is the policy," while accusing O'Rourke of being unable to specifically answer how he'd carry out his mandatory policy to get every AK-47 and AR-15 off the streets.

O'Rourke, in turn, said he believes that Americans will follow the law, no matter the provision. "We don't go door to door to do anything in this country to enforce the law," he said. "The expectation is that Americans will follow the law. I believe in this country. I believe in my fellow Americans. I believe they will do the right thing."

Buttigieg hit back and called attention to O'Rourke's lack of details.

"Well, congressman, you just made it clear that you don't know how this is actually going to take weapons of the streets. If you can develop the plan further, I think we can have a debate about it. But we can't wait. People are dying in the streets right now," he said. "We have to just get something done."

O'Rourke said policymakers should follow the suggestions of the students who led the March for Our Lives movement and the leaders behind Moms Demand Action. But Buttigieg pushed back, saying he hopes every Democrat on that stage recognizes that the common problem is the National Rifle Association "and their enablers in Congress," rather than the varying ideas by the Democratic hopefuls on how to handle gun violence.

Less than three weeks before the debate, Sanders suffered what turned out to be a heart attack.

The 78-year-old senator — the most senior candidate in the race — abruptly canceled his stops on the trail and underwent a procedure for an artery blockage, his campaign said.

When the subject came up, Sanders was quick in trying to assure the audience that his health was not an issue. "I'm healthy, I'm feeling great," Sanders said to a round of applause.

"We are going to be mounting a vigorous campaign all over this country," he continued. "That is how, I think, I can reassure the American people. But let me take this moment, if I might, to thank so many people from all over this country, including many of my colleagues up here, for their love, for their prayers, for their well-wishes. And I just want to thank you from the bottom of my heart, and I'm so happy to be back here with you this evening."

This Saturday in Queens, freshman Rep. Alexandria Ocasio-Cortez, a fellow democratic socialist, will endorse Sanders' bid for the presidency.

The president's Twitter account often sets the day's political agenda. On Tuesday, it formed the basis of a head-to-head between Warren and California Sen. Kamala Harris, who called on her colleague to urge Twitter to take down the president's account.

"I just wanted to say that I was surprised to hear that you did not agree with me that on this subject of what should be the rules around corporate responsibility for these Big Tech companies, when I called on Twitter to suspend Donald Trump's account, that you did not agree," Harris said. "I would urge you to join me."

But Warren did not seem interested in discussing the matter.

"Look, I don't just want to push Donald Trump off Twitter. I want to push him out of the White House. That's our job," Warren said.

"So join me. Join me in saying that his Twitter account should be shut down," Harris responded.

To that, Warren responded: "No."

"No?" Harris asked.

Warren then pivoted to "why it is that we have had laws on the books for antitrust for over a century and yet for decades now we've all called out how the big drug companies are calling the shots in Washington."