By STEVE PEOPLES
AP National Political Writer

WESTERVILLE, Ohio (AP) - Joe Biden is facing baseless - but persistent- allegations of wrongdoing overseas that could undermine his argument that he's best positioned to defeat the president. Bernie Sanders is recovering from a heart attack that raised questions about his ability to withstand the vigor of a presidential campaign. And Elizabeth Warren is fending off new scrutiny of her biography.

A dozen Democratic presidential candidates will meet on Tuesday for the most crowded presidential debate in modern history. But it's the three leading candidates - Biden, Sanders and Warren - who face the most intense spotlight that could expose glaring liabilities in their quest for the White House.

The debate marks the first time the candidates will meet since the House moved forward with an impeachment inquiry against President Donald Trump. While they are united on that issue, the debate will again expose a political party struggling to coalesce around a unifying message or messenger with the first primary contest just over three months away. The event, hosted by CNN and The New York Times, will be held in Ohio, a state that has long helped decide presidential elections but has drifted away from Democrats in recent years.

The White House hopefuls will represent the political and personal diversity that has come to define the Democratic Party of 2019: four women, four people of color, an openly gay man and an age range that spans four decades.

Sanders, a Vermont senator, will be under pressure to prove he has the physical and mental stamina to stand on a podium for three hours less than two weeks after suffering a heart attack. Age was already a concern for the 78-year-old even before he was taken to a Nevada hospital earlier this month and had two stents put in to clear a clogged artery in his heart.

Sanders and his allies insist the health scare has only strengthened his commitment to the 2020 contest - and his case for his signature health care plan, "Medicare for All." Sanders plans to note, as he has in recent days, that millions of Americans without health insurance could have been forced into bankruptcy - or worse - under the same circumstances.

"Bernie is a political marathon runner," said Sanders' confidante RoseAnn DeMoro, the former executive director of National Nurses United. "I think the debate is going to be a great reassurance to the public."

The stakes are also high for Warren, Sanders' ideological ally, who stands on the debate stage for the first time as a front-runner, a status that makes her a top target of rivals in both political parties.

Biden, 76, and others have jabbed her intense focus on detailed liberal policies that may be difficult to implement with a divided Congress. South Bend, Indiana, Mayor Pete Buttigieg has highlighted the absence of a specific Warren health care plan; she has embraced Sanders' single-payer plan instead.

Republicans, meanwhile, have raised questions about whether Warren, 70, 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 issue is particularly threatening because it's not the first time she has been accused of embellishing her biography. Warren, a Massachusetts senator, has already apologized for claiming Native American heritage decades ago. While it showed distant tribal ancestry, it also sparked a rebuke of Warren from some Native Americans for attributing tribal membership to genetics.

"The good news is that most attempts to attack Warren actually make her stronger," said Adam Green, a Warren supporter and co-founder of the Progressive Change Campaign Committee. "Questioning whether she got fired for being pregnant ended up increasing her breadth and intensity of support, particularly among women."

Biden is no stranger to the pressure of front-runner status. But he enters his fourth debate under the weight of intensifying questions from Trump about his family's work overseas.

Without proof, Trump has repeatedly said that Biden's 49-year-old son, Hunter, improperly profited from work in Ukraine and China while the elder Biden was vice president. Trump also insists that Biden used his office to protect his son from allegations of wrongdoing.

There is no evidence to any of the claims, and some of Biden's Democratic rivals have defended him against Trump's baseless attacks. But he will almost certainly be asked by the debate moderators to defend himself.

Hunter Biden will participate in a series of interviews on ABC in the hours before the debate.

Biden addressed the situation over the weekend, promising to bar family members from occupying any office within the White House if elected. But he has yet to address Trump's allegations facing a national debate audience.

How he fares could determine whether he maintains his place atop the crowded field.

Many other candidates, meanwhile, are fighting for their very political survival.

The debate will also feature Buttigieg, California Sen. Kamala Harris, New York entrepreneur Andrew Yang, former Texas Rep. Beto O'Rourke, Minnesota Sen. Amy Klobuchar, New Jersey Sen. Cory Booker, former Obama housing chief Julián Castro, billionaire activist Tom Steyer and Hawaii Rep. Tulsi Gabbard.

With the Iowa caucuses scheduled for Feb. 3, time is quickly running out for the lower-tier candidates to make a move. Based on current polling, several wouldn't qualify for the next round of debates.

The risks offer a compelling reason for the struggling White House hopefuls to be more aggressive.

Buttigieg has squabbled with O'Rourke recently over guns, and he signaled over the weekend that he planned to make a sharper distinction between himself and Warren. Health care emerged as a key contrast among candidates in recent debates and it likely will again, at least according to the 37-year-old mayor, whose plan would give people a choice of whether to join a government-backed health care system or their private insurer.

Speaking to a labor union audience at a presidential candidate forum in Iowa, he warned that some of his competitors, without explicitly naming Warren, would give Americans no choice.

"I'm not pushing Medicare for All whether you want it or not," Buttigieg said.

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Associated Press writer Thomas Beaumont in Altoona, Iowa, contributed to this report.