WESTERVILLE, Ohio — Sen. Elizabeth Warren, whose steady stream of plans have propelled her to the top of the Democratic presidential field, came under sustained attack for the first time during a raucous primary debate here Tuesday that revealed Democrats’ ongoing public struggle to define their beliefs and choose a standard-bearer to take on President Trump.

Warren (Mass.) faced direct challenges from several of the 11 other candidates onstage, who took her on over her policies, her fitness to serve as commander in chief and her willingness to question the motives of Democrats who support less transformational ideas.

South Bend, Ind., Mayor Pete Buttigieg and Sen. Amy Klobuchar (Minn.) took turns criticizing Warren over her support for Medicare-for-all and her purist approach to populism.

“We’re competing to be president for the day after Trump. Our country will be horrifyingly polarized, even more than now. After everything we’ve been through, after everything we are about to go through, this country will be even more divided,” Buttigieg said. “Why unnecessarily divide this country over health care when there’s a better way to deliver coverage for all?”

Sen. Amy Klobuchar (D-Minn.) attacked Sen. Elizabeth Warren (D-Mass.) for promoting what she said was an unrealistic health-care plan during the Oct. 15 debate. (CNN)

The focus on Warren deflected attention from others facing scrutiny entering Tuesday’s debate, which was hosted by CNN and the New York Times.

Former vice president Joe Biden went for long stretches without surfacing, as the rest of the field appeared reluctant early in the debate to echo Republican criticisms of Biden’s son, Hunter, and his work on the board of a Ukranian energy company while his father was the Obama administration’s point person there. Attacks on Biden’s more moderate positions were fleeting, unlike in earlier debates. 

Sen. Bernie Sanders (I-Vt.), meanwhile, showed no signs of lingering health trouble in his first public appearance since suffering a heart attack two weeks ago. But he also did not answer questions about his health.

The stage was swollen with a dozen candidates, the most on a debate stage this year. And with several at risk of not making the next set of debates, a number of feisty exchanges were triggered by low-polling candidates still hoping to revive their campaigns. The debate also came at a pivotal time: just four months before the Iowa caucuses, and when the impeachment inquiry that House Democrats have launched against Trump has sucked up most of the political oxygen.

It was the unrelenting onslaught against Warren that most deviated from past gatherings.

Former congressman Beto O’Rourke of Texas accused Warren of supporting “punitive” tax policy. Businessman Andrew Yang questioned Warren over her plan to break up large technology companies. Sen. Kamala D. Harris (Calif.) chastised Warren for not agreeing that Trump’s Twitter account should be suspended. Unprompted, Rep. Tulsi Gabbard (Hawaii) twice roped Warren into her answers, questioning the senator’s foreign policy credentials.

“I appreciate Elizabeth’s work,” Klobuchar said at one point. “But again, the difference between a plan and a pipe dream is something that you can actually get done.”

The tenor of the debate was the clearest sign yet that Warren’s ascendancy would force her campaign to grapple with the challenges of being the front-runner in a large and unwieldy Democratic field.

Warren fought back against the attacks, casting her challengers as too small-minded or timid for the moment, or as part of an entrenched political system that is dominated by corporations and the wealthy.

“Look, I understand that this is hard, but I think as Democrats, we are going to succeed when we dream big and fight hard, not when we dream small and quit before we get started,” Warren said. 

The debate also underscored the deep policy divisions that persist among Democrats on issues including health care, abortion, taxes, education, inequality, technology, foreign policy and trade. Health care was perhaps the most contentious issue, as it has been in previous debates. 

Moderate candidates pilloried the Medicare-for-all program backed by Warren and Sanders as overly expensive, unworkable and likely to raise costs on middle-class Americans.

“Your signature, senator, is to have a plan for everything, except for this,” Buttigieg said, facing Warren, as he accused her of giving an evasive answer when asked if Medicare-for-all would lead to tax increases for Americans.

Klobuchar, among those onstage who have yet to meet the qualifications for the November debate, also weighed in.

“At least Bernie’s being honest here and saying how he’s going to pay for this and that taxes are going to go up,” Klobuchar said. “And I’m sorry, Elizabeth, but you have not said that, and I think we owe it to the American people to tell them where we’re going to send the invoice.”

But Klobuchar also challenged Warren’s approach more broadly. More than once, she told Warren that her idea is “not the only idea,” suggesting that Warren’s approach was both inflexible and impractical.

Warren insisted that overall costs for the middle class would go down under her plan, while corporations and the wealthy would pay more. She also criticized plans from more moderate candidates who have pushed for an optional Medicare buy-in program.

“I will not sign a bill into law that does not lower costs for middle-class families,” she said.

Sanders defended his Medicare-for-all plan, which would provide everyone with government-sponsored health care and would largely abolish the private health-care industry.

As the debate began, the moment felt particularly tenuous for Biden, whose campaign has been rattled by attacks from Trump about his son Hunter that have temporarily overshadowed concerns that some Democratic primary voters had previously voiced about a record they view as out of step with the base of the party.

Biden’s lukewarm debate performances and his halting response to Trump have only elevated those concerns, which have become evident in declining poll numbers and a fundraising haul that was eclipsed by all of his major primary rivals. 

In the days leading up to the debate, Biden and his son launched twin efforts to try to alleviate concerns that Biden is poorly positioned to counter Trump’s baseless charges about his son while at the same time answering concerns about why he allowed the perception of a conflict of interest. 

Biden has vowed that his administration would not allow family members to conduct overseas business, a pledge meant to inoculate him from attacks but one that also raises questions about why he did not do more to prevent an appearance of conflict while he was vice president. When asked about that contradiction early in the debate, Biden did not directly respond.

“My son did nothing wrong,” he said. “I did nothing wrong.”

With Democrats reluctant to pile on, the debate turned to other topics, and reached its caustic peak when O’Rourke and Buttigieg, whose tensions have boiled just below the surface, engaged in a bitter clash over gun buyback programs. O’Rourke is one of the few candidates to push for a mandatory gun buyback program, which Buttigieg said was unrealistic. 

“Congressman, you made it clear you don’t know how this would work,” Buttigieg responded. “Develop the plan further, we can have a debate.”

“This is not a purity test,” O’Rourke said. “This is a crisis, and we’ve got to do something about it.” He suggested that Buttigieg was adopting poll-tested positions and lacking the political courage to do something bold. 

“I don’t need lessons from you on courage, political or personal,” Buttigieg snapped. 

It was part of a more forceful performance from Buttigieg, who directly challenged several of the contenders onstage, often turning to face them as he delivered pointed barbs.

At one point, he turned to Gabbard, a fellow combat veteran, and told her she was “dead wrong” to say the United States should immediately withdraw troops from the Middle East. 

That exchange came during a lengthy discussion of foreign policy, which has played a limited role thus far in a presidential primary dominated by domestic issues like health care, inequality and race.

Trump’s recent decision to withdraw troops from northeast Syria and allow a Turkish invasion has drawn bipartisan condemnation and raised questions about how Democratic presidential candidates would handle the country’s foreign entanglements.

The Trump administration is struggling to contain the damage in the wake of Turkey’s incursion, with White House officials saying that it has led to a humanitarian crisis and increased the threat of terrorism.

Although Gabbard and Buttigieg have each called for the United States to bring troops home from the Middle East, they took opposing positions on the stage Tuesday.

“What is an endless war if it’s not another regime-change war?” she asked Buttigieg.

The United States has been at war in Iraq and Afghanistan since the early 2000s, and Biden was among those who voted to authorize the conflicts. But in a sign of Biden’s relatively drama-free night, his vote did not come up during the lengthy discussion of foreign policy. Sanders referred to it glancingly toward the end of the debate, calling the vote “disastrous.”

Candidates instead went fiercely after Trump, whom Booker accused of “moral weakness” and a “dumpster fire” policy. Biden called Trump an “erratic, crazy president who knows not a damn thing about foreign policy.”

Castro put it in even blunter terms: “This president is caging kids on the border and effectively letting ISIS prisoners run free,” Castro said.