Democratic rivals sprang against Sen. Elizabeth Warren at Tuesday night’s presidential debate when she repeatedly refused to say whether she’d raise taxes on the middle class to pay for her Medicare for All health insurance plan. It was the most striking confrontation so far among the candidates to take on Republican President Donald Trump next year.

Warren, pressed as to whether taxes would go up to pay for her plan, responded, “So, I have made clear what my principles are here. And that is costs will go up for the wealthy and for big corporations, and for hard-working middle-class families costs will go down.” But she would not say the same for taxes.

More moderate Democrats — including Mayor Pete Buttigieg of South Bend, Indiana, and Minnesota Sen. Amy Klobuchar pushed for a more restrained approach that would stop short of fully government funded health care.

“I appreciate Elizabeth’s work but, again, the difference between a plan and a pipe dream is something you can actually get done,” Klobuchar said. Added Buttigieg: “We heard it tonight. A yes or no question that didn’t get a yes or no answer.” He said he wanted a plan that could be summed up as Medicare for All if you choose it, not whether you want it or not.

The exchanges played out during the Democrats’ fourth debate, the 12 candidates lining up as the most ever on one stage. Warren’s emerging status as a leading candidate could be measured by the attacks on her policies ranging from Medicare for All to her questioning of the role of automation in job losses.

Former Vice President Joe Biden also faced questions about Trump’s baseless charges that he and his son profited improperly from overseas work.

“My son did nothing wrong. I did nothing wrong,” Biden said. “My son made a judgment. I’m proud of the judgment he made,” he said before pivoting to Trump’s “corruption.”

Hosted by CNN and The New York Times, the debate came as the impeachment inquiry against Trump has grabbed most of the nation’s political attention and left the Democratic race for the White House somewhat overshadowed.

It took place on the campus of Otterbein University, just outside Columbus in Ohio, one of the nation’s toughest electoral battlegrounds, but also a state that has long helped decide presidential elections but has drifted away from Democrats in recent years.

Without providing evidence, Trump has repeatedly said that Biden’s son improperly profited from work in Ukraine and China while the elder Biden was vice president. The president also insists that Biden used his office to protect his son from allegations of wrongdoing.

Biden’s Democratic rivals have united in criticizing Trump, but the 76-year-old former vice president himself has been pushed to provide explanations.

So is Warren, 70, who, in addition to health care, is also facing questions from opponents about if she was actually forced from her teaching job because of a pregnancy nearly a half century ago, a claim that has become a core part of her personal message. Critics have pointed to past speeches and documents that suggest she left on her own. Warren is standing by her story.

The 78-year-old Sanders, meanwhile, spent most of the last two week recuperating in Vermont after doctors inserted two stents put in to clear a clogged artery. He’s using the debate to show he’s fully recovered before formally rejoin the race with a rally in New York on Saturday.

Buttigieg and California Sen. Kamala Harris are trying to crack the field’s top tier.

Also debating are New York entrepreneur Andrew Yang, former Texas Rep. Beto O’Rourke, Minnesota Sen. Amy Klobuchar, New Jersey Sen. Cory Booker, former Obama housing chief Julian Castro and Hawaii Rep. Tulsi Gabbard. Making his debate debut is billionaire activist Tom Steyer.