On Biden, Trump and Ukraine 2020 Democrats walk a fine line, as they eye the field’s current front-runner status

Though Joe Biden’s 2020 Democratic rivals have previously treated President Donald Trump's unsubstantiated claims of the former vice president's inappropriate behavior surrounding his son Hunter’s Ukrainian business dealings with kid gloves, many of the candidates vying for a spot on the 2020 ticket now have launched thinly-veiled jabs against the veteran lawmaker.

Some in the crowded field have maintained full-throated support, while in the same breath saying they themselves would never allow their cabinet members’ families to sit on a foreign board, as Hunter Biden did during the Obama administration.

It’s a subtle attack strategy which lets fellow Democrats telegraph that perhaps their moral compass’ north is finer-tuned than Biden’s -- casting doubt on his political viability -- while still keeping up appearances of united coalition.

This type of tightrope walking play out further as the field convenes for the fourth Democratic primary debate.

"I think we'll see the defense really dramatized," Democratic strategist Arshad Hasan told ABC News. "The attacks on him in debate three didn't work as well as the others thought it would. Now, candidates will want to highlight that we have the obligation to something greater than ourselves. They'll want to prove, 'I'm a statesman' -- and that's a powerful contrast on stage -- and, with President Trump."

Former Texas congressman Beto O’Rourke recently walked that fine line in comments to reporters.

“I would not allow a family member, anyone in my cabinet, to have a family member to work in a position like that,” he said, adding, “There is no reason to pursue an investigation in Joe Biden or his son Hunter Biden. They have been cleared of any wrongdoing whatsoever -- the focus has to remain on Donald Trump and the crimes that he’s committed.”

Sen. Elizabeth Warren, D-Mass., when asked if, under her anti-corruption plans, her vice president's child would be allowed to serve on a foreign company's board of directors, she answered “No. I don’t - I don’t know - I mean, I’d have to go back and look at the details of the plan."

Such responses come as President Trump lobs attacks at Biden and his son -- moving Democrats to circle their wagons around the former vice president.

It also forces them to walk a fine line, as they eye the field’s current front-runner status.

"No one is obligated to defend the front-runner," Hasan told ABC News. "But here - think about what would happen if they piled on - if they used Trump's attacks as their own."

"There's a genuine disgust, and sense of existential threat - I actually think Democrats would not be so united if it were just Trump's attacks - but here, the machinery of state is being used to dig up dirt on political opponents," Hasan said. "They're standing their ground for the rule of law... and if that presents the opportunity for contrast with their rival, that scores bonus points."

Biden pressured the Ukrainian government to oust a prosecutor who ostensibly had been leading an investigation into Burisma, an oil company, and was unpopular in his home country due to a lack of action. However, no evidence has emerged to support Trump’s main allegation that Biden did so to benefit his son, Hunter, who was later added to the company's board of directors. Several international leaders, including senior officials at International Monetary Fund, have criticized the prosecutor and said Biden’s recommendation was justified.

“It certainly has a bad look to it,” Andrew Yang on having a vice president's child sit on the board of a foreign company at a recent campaign stop. “In my mind they can wait until the term is over before serving that term. And that’s really the way it would be under my administration.”

Yang later clarified that his comments were not a hinted jab at Biden.

“If you look at when he was vice president, were there any rules against your family members being on the boards of foreign companies? I believe there were not. So nothing was amiss,” he said.

Yang's positioning typifies the type of position the Democratic presidential field finds itself in.

"I think they're worried that there's a question in some people's minds about whether this was above board or not," said Yvette Simpson, CEO of Democracy for America and an ABC News political contributor. "And I think they want to be clear that they would steer far far away from that... they want to be careful. But they also want to be clear."

Biden's core campaign pitch of electability ups the ante on optics -- making them more important than they would already be - and making navigation forward now for him all the more challenging.

"The more you look like your opponent, the less likely there's a distinction that you can use to your advantage on election day," Simpson told ABC News. "And the more Joe Biden appears like Trump -- not that he is, but if there's a question about that -- the more these things rack up - the harder it's going to be for him to challenge Trump, and really draw that line and say, well, you don't have to worry about that with me."

Minnesota Sen. Amy Klobuchar was asked on CNN this month if she’d allow her vice president’s child to sit on a foreign board.

“No, I wouldn’t,” she said. “And I can promise you right now my own daughter, who is only 24, does not sit on the board of a foreign company.”

She then quickly pivoted back to Trump. “But that is not the issue, the issue here is what the president is doing… there’s no evidence the vice president did something wrong here.”

And amid continued affirmations their primary competitor did nothing wrong, they are drawing a bright-line that they would behave differently were they elected to higher office.

"I think it’d be better not to have that kind of an arrangement, and it’d be better not to give speeches to Goldman Sachs," Colorado Sen. Michael Bennet said in a recent interview with Politico. "It’d be better for people just to be public servants.”

Sen. Kamala Harris, D-Calif., said last month she would “probably not” be comfortable with what Biden’s son did in her own administration. Despite the many sharp words she’s shot on and off stage at the former Vice President– on this, she’s kept them in her quiver.

“You know, leave Joe Biden alone,” Harris told reporters recently. “I’m not going to get seduced into what Donald Trump is trying to get people to do.”