WESTERVILLE, Ohio -- Sen. Elizabeth Warren finished first in a national poll on Monday and was quickly the focus of attack on Tuesday as Democrats vigorously debated for three hours in battleground Ohio.

The Massachusetts senator took fire for refusing to say whether middle-class Americans' taxes would increase under her "Medicare for All" plan and for a "wealth tax" that she and Vermont Sen. Bernie Sanders both have endorsed.

"I will not sign a bill into law that does not lower costs for middle-class families," she said, later suggesting that candidates attacking her plan to tax the most wealthy Americans are seeking to protect billionaires.

Perhaps the only 2020 candidate to face more attacks than Warren at Otterbein University was President Donald Trump. All the candidates kicked off the night with calls for his impeachment, with some calling him the most corrupt president in the nation's history as each fought to make their mark and survive to the next round of debates.

Asked directly about his health after suffering a heart attack this month, Vermont Sen. Bernie Sanders quickly replied, "I'm healthy. I'm feeling great." He added that he will be "mounting a vigorous campaign all over this country."

Former Vice President Joe Biden aggressively defended himself and his son Hunter from unsubstantiated allegations by Trump and his GOP supporters of misdeeds in Ukraine and China.

"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. That's what we should be focusing on," Biden said.

The evening also featured a couple of candidates proposing legalizing opioids for personal use, universal support for abortion rights and a biting exchange over regulating guns between South Bend Mayor Pete Buttigieg and former Texas Congressman Beto O'Rourke, who proposes a mandatory buyback of so-called assault weapons such as the AK 47 and AR 15.

But it was Warren — not Biden, as in past debates — who was singled out for criticism by her fellow Democrats.

"Tonight Elizabeth Warren found out what it feels like to be the Democratic frontrunner on a debate stage and the results were a mixed bag," said Aaron Kall, director of debate at the University of Michigan. "She was quickly attacked over middle tax increases that could accompany her Medicare for All proposal, but she unfortunately remained evasive and never fully answered the important question."

Warren's dodge on the Medicare for All interchange was quickly noted by Buttigieg, who said she was playing the typical Washington game of "a yes-or-no question that didn't get a yes-or-no answer."

Buttigieg pushed his "Medicare for all who want it" proposal, saying it doesn't kick some 150 million Americans off their private health insurance. "If you prefer to stay on your private plan, you can," he said.

Warren countered that the Indiana mayor was actually proposing "Medicare for all who can afford it." She added, "Medicare for All is the gold standard. It is the way we get coverage for every American family."

Sanders, whose plan Warren has endorsed, interjected, "As somebody who wrote the damn bill, let me be clear."

Sanders said while taxes will go up for many Americans, "the overwhelming majority will save money on their (health-care) bills," and wind up financially ahead overall.

When asked if she would accept Sanders' explanation of taxes going up but most people saving money, Warren again declined to answer directly.

That brought a rejoinder from Minnesota Sen. Amy Klobuchar: "At least Bernie's being honest here in saying how he's paying for it."

Warren is neglecting to tell Americans "where we're going to send the invoice" for Medicare for All, Klobuchar said. She advocated her plan to give consumers a public option where they could sign onto a government insurance plan or keep their private plan.

Instead, Kobuchar told Warren, "You are making Republican talking points right here in this room. ... The difference between a plan and a pipe dream is something you can actually get done."

Biden said the plans by Warren and Sanders are "going to cost over $30 trillion in 10 years." And many Americans will not enjoy a net decrease in costs, as Warren and Sanders maintain, Biden said.

Many had predicted an especially vigorous debate Tuesday night as leading contenders in the middle of the platform — Biden, Warren and Sanders — try to gain ground over their main competitors for the nomination. But rhetorical bombast came from the fringe of the stage as well during the event, moderated by CNN's Anderson Cooper and Erin Burnett and The New York Times' Marc Lacey.

Klobuchar attacked Warren over her insistence that a wealth tax was the best way to pay for some of the large-scale programs she has proposed, saying there is room for more ideas. Warren wants to tax wealth over $50 million to pay for early childhood education, health care and other priorities.

Most of the other candidates said they didn't outright oppose the plan, but they offered other ways to balance income inequality. Harris said she would pursue a tax credit worth $6,000 a year for families earning less than $100,000 a year. Businessman Andrew Yang, whose principle campaign platform has been a universal basic income of $1,000 a month per person, said that wealth taxes haven't worked in other countries.

"Nobody on this stage wants to protect billionaires. Not even the billionaire wants to protect billionaires," Klobuchar said, nodding to former hedge fund manager/impeachment zealot Tom Steyer, the wealthiest candidate on the stage.

Four of the 12 candidates have yet to qualify for the November debate in Georgia: former housing secretary Julián Castro, Hawaii Congresswoman Tulsi Gabbard, Klobuchar and O'Rourke. Steyer qualified for his first debate Tuesday.

Then there were candidates in between trying to force their way into the top tier: New Jersey Sen. Cory Booker, Buttigieg, California Sen. Kamala Harris and businessman Andrew Yang.

The wildest of the wild cards was perhaps Gabbard, who threatened to boycott Tuesday's debate because "the corporate media and political establishment have done everything in their power" to silence her. She took perhaps the most strident stance on removing U.S. forces from Kurdish territory in Syria along the border with Turkey, using her answer to challenge Warren to call for an end to a "regime-change war" that has been raging in Syria for years.

"I don't think we should have troops in the Middle East but we should do it in the right way," Warren retorted, saying Trump's plan to withdraw from the region created a humanitarian crisis and "gave ISIS a new lease on life" in Syria.

Leaving the Kurds to fend for themselves has undermined U.S. credibility among its allies, Sanders said, asking "What country in the world will trust the word of the president of the United States?"