As a dozen Democrats prepared to face off in the fourth presidential debate Tuesday, polls indicated former Vice President Joe Biden and Sen. Elizabeth Warren of Massachusetts running neck-and-neck and widening their leads in possible matchups against President Trump.

The Real Clear Politics average of recent polls still gives Biden a six-point lead over Warren for the Democratic presidential nomination. The Morning Consult/Politico update released Monday gave Biden an 11-point lead nationally over Warren while Public Policy Polling flipped the result, giving Warren a 12-point lead over Biden in Maine. Sen. Bernie Sanders of Vermont, who suffered a heart attack two weeks ago, came in third in both polls, with 19% support in the Morning Consult poll and 9% according to PPP. A Sienna poll of New York voters indicated Biden and Warren tied at 21%, with Sanders trailing at 16%.

The PPP poll indicated Biden would beat Trump by 12 points, and both Warren and Sanders would beat Trump by 10 points.

A Georgetown University Politics Battleground poll indicates 56% of Americans have a negative view of Trump and half said they would vote for a Democrat over Trump.

Trump has been working hard to damage Biden’s candidacy, promoting unsubstantiated allegations of corruption involving Biden’s son Hunter’s foreign business ventures to turn attention away from the House impeachment inquiry.

Tuesday night’s scheduled three-hour debate was set to begin at 8 p.m. EDT at Otterbein University in Westerville, Ohio, just outside Columbus.

With 12 Democratic presidential hopefuls participating, the debate is being billed as the largest presidential debate ever. Joining the top three frontrunners are Sen. Kamala Harris of California, South Bend (Ind.) Mayor Pete Buttigieg, former Rep. Beto O’Rourke of Texas, Sen. Amy Klobuchar of Minnesota, businessman Andrew Yang, Sen. Cory Booker of New Jersey, former Housing Secretary Julian Castro, Rep. Tulsi Gabbard of Hawaii and businessman Tom Steyer, a late entry into the voting sweepstakes who must explain why he’s running.

Warren is seen as the candidate with the most momentum going into the contest. She has been piling up millions more in campaign donations than Biden but still faces questions about the effects of Medicare-for-all on taxpayers.

Until recent days, Biden has pretty much ignored the unsubstantiated allegations Trump and his personal attorney, Rudy Giuliani, have been peddling. Biden’s son gave an interview to ABC that was released Tuesday in which he denied any wrongdoing but admitted his name probably opened doors. A Monmouth University poll indicates the disinformation campaign has some voters wondering if there’s anything behind the allegations. Biden needs to convince voters he’s up to the task of taking on Trump.

Sanders, the oldest of the candidates at 78, needs to show voters he’s healthy. His campaign has not answered questions about the seriousness of his heart attack, and an appearance at the debate would be his first public appearance since the incident.

Harris needs a repeat of her first debate performance when she scored by attacking Biden’s position on school busing decades ago. Her polling since then, however, has fallen into single digits.

The rest of the field also needs a viral moment if the candidates expect to survive into primary season.

A fifth debate is scheduled for Nov. 20 in Georgia.