As Biden weathers Trump's criticism on Ukraine, supporters worry about possible impact on candidacy

Joe Biden's efforts to fend off President Donald Trump's claims of the former vice president's inappropriate behavior and his son Hunter’s Ukrainian business dealings during the Obama White House threaten to undercut his campaigns's central argument that he is the sole Democrat who can oust Trump from the Oval Office and restore a sense of normalcy to a country deeply bruised by partisan divisions.

Amid a tightening primary contest, his campaign has found itself rebutting the president’s attacks, while simultaneously staying on message to keep the persistent candidacies of his two main rivals for the Democratic presidential nomination, Massachusetts Sen. Elizabeth Warren and Vermont Sen. Bernie Sanders, at bay.

The war of words that reached a new level of intensity on Wednesday during a campaign speech in Rochester, New Hampshire when Biden said firmly that he believes Trump should be impeached.

"To preserve our Constitution, our democracy, our basic integrity, he should be impeached," Biden said, painting Trump's attacks as solely motivated by politics, and called the president a "bully" and a "coward" that is afraid to face him in a general election match-up.

The line sparked an immediate back and forth between Biden and Trump on Twitter.

Despite the ongoing barrage of attacks from the president, and lackluster fundraising numbers in the third quarter of 2019, Biden remains at or near the top of the Democratic pack in most public polling with less than four months until the first votes of the 2020 race are cast.

Fending off Trump's attacks

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.

Still, the president's near-constant attacks on Twitter and on the airwaves, including a more than $1 million ad buy from President Trump’s re-election campaign this past weekend targeting Biden in the early nominating states of Iowa, New Hampshire, Nevada and South Carolina, and Biden’s at times uneven response, have fueled worries even among supporters who believe he is handling the situation the right way.

“He's had difficulty coming up with a single strategy,” former Senate Majority Leader Tom Daschle, a Biden supporter who said he still believes the former vice president to be the strongest general election candidate against Trump, told ABC News.

“He sounds quite defensive, and that's somewhat troubling. He's probably unavoidably been drawn into this because of the circumstances...because of his personal connection to the story and his natural inclination to be very defensive about his family. I think it's complicated his response and his strategy a great deal,” Daschle, who served alongside Biden for his entire 18-year career in the U.S. Senate, added.

Others also expressed concern that attempting to go tit-for-tat with Trump will not be a winning strategy for Biden.

“Engaging Trump gives Trump what he wants: the ability to rope-a-dope Biden, to get him on the ropes, and strangle him. This needs to be about Donald Trump, not Joe Biden,” South Carolina state Sen. Dick Harpootlian, a former chair of the state Democratic Party and one of Biden’s most prominent backers in the crucial first in the south state, told ABC News.

Harpootlian warned that engaging with Trump directly - as he continues to escalate his claims - comes with significant risks and a potentially unpleasant cost.

“I was reminded the other day of sage advice given by Fritz Hollings to both me and Joe,” Harpootlian said, referencing the late South Carolina Senator who was a close friend of Biden during their more than three decades together in the U.S. Senate. “What Fritz would say is this: 'Never wrestle with a turd because you're going to get up smelling like s---.’”

Harpootlian, who said he firmly believes Biden will win South Carolina when the state holds its primary on February 29 of next year, said that regardless of the difficulties presented by Trump’s attacks, the vice president is handling them “extremely well.”

“Donald Trump's doing a great job of killing himself. Joe Biden's doing a great job of staying out of the way,” he said.

Daschle, who led the Senate Democratic caucus during President Bill Clinton’s 1999 impeachment trial, warned that while Biden’s personal connection and need to defend his family may be driving his response, it is unwise to continue to engage the president on a regular basis.

“I really don't think he benefits at all from being drawn into the day-to-day give and take and the day-to-day exchange that Trump would love. Trump would love to engage him on a daily basis, to make this a Trump-Biden scandal,” Daschle said.

“He ought to be succinct, he ought to stay on message, and then pivot to the things he wants to...focus on,” Daschle added. “It seems to me, he ought to invite scrutiny and just say: 'Look, let them have it...they can look as much as they want to.’ Invite that, make his point about Trump, and then move on. But I think it's really critical, and this is hard for Joe sometimes to do, to make that pivot and to get on to the things he wants to talk about.”

It’s a sentiment echoed by political experts who warn that Biden faces a dual test - focus and hone his own campaign’s message without letting Trump control the narrative.

“It's a Hobbesian choice. What do you do? Do you try and make sure that people hear your side of it, which negates his side of it, but also spreads the information further? Or do you try to focus on other things and slough it off, like water off a duck's back. That's the line they've been treading,” Larry J. Sabato, the founder and director of the University of Virginia Center for Politics, told ABC News.

It’s a choice the Biden campaign argues that they are well-equipped to handle.

"We are not going to let [Trump] turn the conversation away from the corruption and malfeasance that is impacting daily voters' lives, we're going to be very tough about that. But simultaneously, we're going to remind people why Donald Trump is doing this, and it's because he fears Joe Biden," Biden's Deputy Campaign Manager Kate Bedingfield argued on CNN Tuesday.

The public war of words between Trump and Biden could be the opening salvo in what promises to be a bitter general election campaign to deny the Republican a second term in the Oval Office.

“[Biden’s] main selling point...has been that he's the guy who will take on Donald Trump in a fight. And it's going to be a dirty fight. And Donald Trump just threw the first punch. I think Democratic voters are going to take a close look at Joe Biden and say, how does he punch back?” Patrick Murray, director of the Monmouth University Polling Institute, told ABC News.

For his part, Biden is showing no signs of backing down, even as Trump’s attacks against him have escalated in the last week, so too has the ferocity of his response.

First, Biden gave a speech in Reno, Nevada where he offered his clearest and most personal rebuke to date to Trump’s allegations against him and his family.

“You are not going to destroy me, and you’re not going to destroy my family. I don’t care how much money you spend, Mr. President, or how dirty the attacks get,” Biden said in a speech that, while it was well-received, was given past 10 p.m. on the east coast -- missing the cut for prime time coverage.

Then on Friday, after President Trump publicly urged China to pursue an investigation into him and his son on the south lawn of the White House, Biden gave a fiery statement to reporters following his appearance at a forum hosted by the Services Employees International Union (SEIU) in Los Angeles.

“All this talk from the President about corruption comes from the most corrupt president we’ve had in modern history. He is the definition of corruption,” Biden said, later re-iterating a previous statement that he has not spoken to his son about overseas business dealings and denying the idea that his son’s employment by a Ukrainian gas company while he led the United States’ diplomatic efforts there presented a conflict of interest.

A rising challenge from the left

Aside from the challenge of counter-messaging Trump, Biden is also contending with Warren’s consistently rising campaign, and voters who are not firmly loyal to the former vice president.

“What's problematic for [Biden] right now is that we're seeing a trend that has already been developing, where many of those voters are looking at Elizabeth Warren, who they had originally written off, and are saying, 'You know what, she's not as bad as I thought she was on policy. She's proven herself to be a pretty good fighter.' That's dangerous for Joe Biden,” Murray said.

Biden's 2020 rivals have been reticent to directly attack his diplomatic efforts and his son's employment in Ukraine, but some have made clear they would not allow the arrangement if they were in the Oval 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.”

Warren has not directly criticized Biden on the issue, but has suggested her anti-corruption plan would bar her potential vice president's children from serving on the boards of foreign companies, a role in which Hunter Biden served.

A matter of public perception

In another problematic sign for the former vice president that may show the necessity to counter Trump’s claims more forcefully, 43% of respondents in a recent Monmouth University poll said that Biden “probably did” put pressure on Ukrainian officials to get them not to investigate his son’s business dealings there, despite the lack of evidence supporting the allegation.

As Biden re-emerges back on the campaign trail this week in New Hampshire, the attacks from Trump and his attempt to coax Ukraine into investigating the Democrat and his son, could cement the idea in voters’ minds that the former vice president is still the candidate the president fears the most, Harpootlian argued.

“Trump wasn't focused on Bernie or Elizabeth Warren or any of the others. His decision...his analysis was that his major threat was Joe Biden. And that's why this summer he's calling the Ukrainian President...trying to stir up dirt on Joe and Hunter,” Harpootlian said.

For his part, Trump has said his call for foreign governments to investigate Biden and his son was related to corruption, not politics. After previewing it on Twitter last week, the president repeated that message multiple times in a 23-minute back-and-forth with reporters, a shift from his days of disparaging a whistleblower whose complaint about his dealings with Ukraine sparked the impeachment inquiry process debate.

"I'm only interested in corruption," Trump told reporters at the White House in comments that drew a sharp rebuke from Utah Republican Sen. Mitt Romney. "I don't care about politics. I don't care about Biden's politics."