(Bloomberg) — Joe Biden has a warning for his Democratic rivals as they prepare for the fourth televised debate next week: Stay away from the issue of Ukraine and Hunter Biden.The 12 candidates participating in Tuesday’s debate in Westerville, Ohio, are honing their answers to questions about the U.S. House impeachment inquiry into President Donald Trump, which centers on the president’s request to the leader of Ukraine to investigate unfounded allegations about Biden and his son’s work there.But the front-runner’s campaign suggests they shouldn’t stray into shots about the Biden family.An aide to Biden said that any candidate who “calls themselves a ‘Democrat’” and repeats what the aide said were “discredited lies” about Biden and his son “would be making a profound statement about themselves.”As Trump has escalated his attacks against Joe and Hunter Biden, Biden’s Democratic rivals have been walking a tightrope. All of them are campaigning against Trump and most have called for his impeachment. Yet getting the chance to challenge the incumbent president requires first winning the nomination. So many of the candidates have criticized Biden for “allowing” Hunter Biden to serve on the board of Burisma Holdings, a Ukrainian gas company, while his father was vice president, saying any appearance of a conflict of interest is problematic.Biden has thoroughly disputed any allegations, and no evidence suggests that there was any impropriety when he worked in concert with other Obama administration officials and European allies to recommend that Ukraine’s national prosecutor, Viktor Shokin, be fired.Aides to some of the other candidates in the debate say they do not expect their candidates to shy away from repeating their public criticisms.Senator Kamala Harris of California, who sparred with Biden in the first debate over his past positions on busing, said she would “probably not” let a child of her vice president sit on the board of a foreign energy company.Senator Cory Booker of New Jersey told CNN last month, “I just don’t think children of vice presidents, presidents during the administration should be out there doing that.”After a labor forum in Los Angeles last week, former Representative Beto O’Rourke said, “I would not allow a family member, anyone in my cabinet to have a family member, to work in a position like that.”However, one spokesman for a Democratic debate participant said Democrats have learned from 2016 to tread carefully around using Trump’s attacks on other Democrats because that could sow division. The aide said attacking Biden on the Ukraine issue could result in benefiting Trump in 2020, whoever becomes the nominee.“There will be plenty of other opportunities in this debate to draw policy differences with Joe Biden,” said Adam Green, the co-founder of Progressive Change Campaign Committee and a supporter of Senator Elizabeth Warren of Massachusetts. “When it comes to the issue of Ukraine, I’d expect 100% of the attention to be on impeachment and holding Trump accountable for his corruption.”Still, campaigns are getting ready, with debate moderators eager to pinpoint differences among the candidates.At the same time, the candidates who have most aggressively attacked Biden in past debates have been the ones to leave wounded.In the first debate in July, Harris briefly surged in the polls after eviscerating him for fondly recalling his ability to work with segregationists when he was in the Senate.In August, she, Booker and Julian Castro tried the same tactic and it backfired. Castro made another attempt in the September debate with an aggressive jab at Biden’s age and memory and only succeeded in drawing criticism from some prominent Democrats.Bernie Sanders, a Vermont senator, upbraided Biden early and pointedly on his votes for the North American Free Trade Agreement and the Iraq War. But he, too, failed to gain from it, and surveys have shown him been stuck in the mid-teens since May.The failed attempts to take down a popular rival have made Democratic campaign operatives wary of going negative. Their voters don’t want to see the party bloodied before it faces Trump in the general election.The only Democrat who has consistently gained support since the beginning of summer is Warren, who has actively avoided combat with her rivals.For Biden, the debate will also be a test of his new strategy of aggressive counterattacks. Biden was the last top-tier Democratic presidential candidate to call for Trump’s impeachment, and it wasn’t until Wednesday when he unloaded on the president with a blistering speech in New Hampshire.That speech marked a significant shift in how he responded to Trump’s accusations, but his ability to defend himself while also going on the offensive against will surely be tested on Tuesday.“I’m not going to let him get away with it,” Biden said about Trump on Wednesday. “He’s picked a fight with the wrong guy.”To contact the reporters on this story: Tyler Pager in Washington at email@example.com;Sahil Kapur in Washington at firstname.lastname@example.orgTo contact the editors responsible for this story: Wendy Benjaminson at email@example.com, John HarneyFor more articles like this, please visit us at bloomberg.com©2019 Bloomberg L.P.