The 12 candidates podiums stand ready before the fourth U.S. Democratic presidential candidates 2020 election debate at Otterbein University in Westerville, Ohio, October 15, 2019.Jim Bourg | ReutersTwelve Democratic presidential hopefuls are about to step out onto an Ohio debate stage, and into a new and uncharted phase of the 2020 primary race.The top of the Democratic primary field, once dominated by former Vice President Joe Biden, is in flux. While Biden remains the frontrunner in the polls, his campaign has ceded significant ground to Massachusetts Sen. Elizabeth Warren, who is now beating the former veep according to some key surveys.The House impeachment inquiry into President Donald Trump, which has consumed Capitol Hill, is also expected to take up a sizable chunk of the fourth debate – and it could give Biden a chance to shine, or stumble. The event is being hosted by CNN and The New York Times from 8 p.m. to 11 p.m. ET.Trump’s request for Ukraine President Volodymyr Zelensky to “look into” unsubstantiated corruption allegations against Biden and his son Hunter has turned Barack Obama’s vice president into a key character in the impeachment saga, whether he likes it or not.Trump’s calls for Ukraine — and China — to investigate his possible political opponent in the 2020 election prompted Biden to declare his full-throated support for impeachment just a few days before the debate. All other candidates have supported at least the inquiry into Trump’s impeachment.His son’s denial of any wrongdoing, published Tuesday morning, all but ensures that Biden will have to directly respond to concerns about connections, real or perceived, between Hunter’s business dealings and his actions while serving in the White House.”That is absolutely going to shape some initial conversation in this debate,” said Democratic political consultant Andrew Feldman. “And how does Joe Biden respond? Composed, or angry in a way that works in his favor — or works against him?””I think Hunter was more candid than we’ve seen from any Trump child this whole presidency.”Rebecca KatzDemocratic strategistTrump, his allies and his personal lawyer Rudy Giuliani — described as a central figure in a whistleblower complaint that helped spurred the impeachment efforts — have raised suspicions about Hunter Biden’s employment on the board of Ukrainian natural-gas company Burisma Holdings during Obama’s presidency. They suggest that the elder Biden improperly pressured Ukraine to fire a prosecutor there out of concern that he would investigate the company.No evidence exists that Biden’s actions as vice president were intended to help his son; many Western leaders, in fact, had called on Ukraine to fire that prosecutor over allegations of corruption. Hunter Biden has not been credibly accused of anything illegal related to his work with the company, and unequivocally denied wrongdoing in the ABC News interview that aired in part Tuesday morning.Democratic strategist Rebecca Katz called the interview “a pre-emptive strike” and said Biden will be ready to field questions on the subject at the debate. She also compared the allegations against Hunter Biden to Trump’s own children, some of whom have roles in their father’s campaign and administration even as they pursue business deals.”I think Hunter was more candid than we’ve seen from any Trump child this whole presidency,” Katz said. “That is the precedent, what’s in the White House right now.”Feldman predicts that Biden, who has been attacked by Democrats in previous debates, will show more aggression at the debate in Westerville, Ohio’s Otterbein University.But that strategy carries risk. Biden’s angry response to an attack by California Sen. Kamala Harris in the first debate, for instance, wasn’t well-received among politicos.Warren rises as Biden slipsWarren is “going to need to have her armor on and be ready,” Feldman said. A poor performance could hurt her campaign’s upward trajectory, he said, recalling Harris, whose campaign has lost most of what it gained after her breakout attack on Biden.Sanders, who suffered a heart attack just a few weeks earlier, may also come out swinging in the debate. At 78, Sanders is the oldest candidate in the race, but he has remained a top candidate throughout the primary, as he was in the 2016 race against Hillary Clinton. Biden is 76, and Warren is 70. Trump, meanwhile, is 73.The rest of the fieldBut there are more than three candidates on stage — in fact, there are more Democrats appearing tonight than at any previous debate.Entrepreneur Andrew Yang, Sen. Cory Booker, former Rep. Beto O’Rourke, Sen. Amy Klobuchar and Rep. Tulsi Gabbard will also share the stage alongside Biden, Warren, Sanders, Harris, Buttigieg and Castro.Billionaire Tom Steyer, a late entry in the crowded primary field, will be making his debut debate appearance. “Steyer is a little bit of a wildcard,” Feldman said. “You have to pay attention to some of these fringe candidates, not because they are actually going to win the nomination but because they could change the dynamic.”Yang, the universal basic income advocate, is another one to watch, Katz said. “He raised almost as much as Harris last quarter, and he could raise more than Biden next quarter,” she said.With the Iowa caucuses coming into view, the contenders have started to aim more of their attacks at their primary rivals, rather than just targeting Trump. As the most popular candidates — Biden, Warren and Sanders, most polls show — pull ahead, the pressure on under-performing campaigns to fold rises with each new lackluster fundraising report.Buttigieg, the youngest candidate on stage at 37, recently took swings at the “Medicare for All” health care overhaul supported by Warren and Sanders. And Sanders, a self-described democratic socialist, contrasted himself with Warren, who calls herself a capitalist, in an interview over the weekend.”I am, I believe, the only candidate who’s going to say to the ruling class of this country … ‘Enough with your greed,'” Sanders said over the weekend.