In public, for the last five days, Joseph R. Biden Jr. and his campaign have been on a ferocious offensive, ripping into President Trump for allegedly abusing his powers in asking Ukraine’s leader to help hurt Mr. Biden politically, and castigating the news media for questioning his son Hunter Biden’s financial dealings there. On Tuesday, Mr. Biden made his biggest move yet, saying Congress should start an impeachment inquiry against Mr. Trump if he does not cooperate with their investigations.
In private, though, this has been an enraging and uncertain time for the Biden campaign, and for Mr. Biden himself, who associates say has been astounded both by Mr. Trump’s contact with the Ukrainian government and his obvious enthusiasm for attacking Mr. Biden’s family. He and his advisers have long feared that Hunter Biden would become a target for political opponents, according to people familiar with their thinking, and now the scrutiny of Mr. Biden’s son is bound to intensify.
And as Mr. Trump’s onslaught has pushed the House of Representatives to start an impeachment inquiry, Mr. Biden now faces a crucial political test where his standing may hinge on how he handles a wildly unpredictable confrontation with Mr. Trump.
Bryce Smith, the Democratic Party chairman in Dallas County, Iowa, said the moment showcased Mr. Biden as a candidate Mr. Trump is “genuinely worried about” and offered a reminder of the lengths Mr. Trump may go to oppose any Democratic opponent. But it also exposed Mr. Biden to risks in the leadoff caucus state, he said.
“I could see people who are already on the fence about him, maybe because of his long career in politics, his age, his ability to win this, kind of going, ‘Oh, great, we’re already starting this and he hasn’t even gotten the nomination,’” Mr. Smith said. “And others going, ‘He’s obviously being thought of as a formidable opponent, if Trump is thinking about it like this.’”
Mr. Biden and his campaign team have encouraged the latter interpretation, trying to project an image of strength in the primary and warning the press that the Biden campaign would deliver blunt criticism about coverage it sees as unfair. At a Philadelphia fund-raising event on Monday, Mr. Biden pledged, “I’m not going to take a punch and not punch back.”
Yet allies of Mr. Biden acknowledge privately that the campaign has been girding for months for criticism of Hunter Biden, who has charted a winding career as a bank executive, lobbyist and financier, often operating in parts of the private sector that overlapped with his father’s footprint in government. They have declined to say whether they vetted Hunter Biden’s business activities as part of preparing for the 2020 presidential campaign, arguing that the political world should be solely focused on Mr. Trump’s conduct.
There is no evidence that Mr. Biden, as vice president, intentionally tried to help his son in his dealings with Ukraine, but Hunter Biden’s business career has sporadically drawn attention from political rivals and the media for more than a decade. Associates say Mr. Biden is infuriated by Mr. Trump’s conduct and astonished by the president’s willingness to dismiss some of the most basic ethical lines in American politics.
In Wilmington, Del., Tuesday, Mr. Biden called the president’s actions an “abuse of power’’ that undermined national security, and said he would support impeachment if Mr. Trump did not provide information related to congressional investigations.
In some respects, Mr. Trump’s conduct has reinforced the central message of Mr. Biden’s appeal to Democratic voters: that the country is threatened by an out-of-control presidency, and that ousting Mr. Trump is more important than any other policy goal Democrats might share.
At fund-raisers and other events, Democrats who have spoken with Mr. Biden in recent days say he has largely appeared measured, greeting old friends in his typical effusive style and keeping the focus on his concerns with Mr. Trump’s foreign policy, rather than dwelling on the challenges the moment raises for his family.
Still unclear in this moment of crisis is what conclusions Democratic voters might make about Mr. Biden as an opponent for Mr. Trump, and how a drive for impeachment in Congress might shape the Democratic primary campaign.
Former Gov. Tom Vilsack of Iowa, who saw Mr. Biden at a fund-raiser in Iowa on Saturday, said he believed that Mr. Trump’s dangerous conduct was at the forefront of Mr. Biden’s mind.
“I suspect he’s just as focused and just as concerned about that as he is about an attack on a family member, as personal as that is,” said Mr. Vilsack, who served as agriculture secretary in the Obama administration.
A memo circulated to political surrogates from the Biden campaign on Tuesday urged supporters to note that “this isn’t about Biden or his family. We all know who Trump is,” and also suggested criticizing reporters for giving undeserved coverage to Mr. Trump’s attacks on Mr. Biden and his son.
“Trump is successfully playing the same game he played in 2016 — and the press is falling for it again,” the memo stated.
This clash has come at a precarious moment for Mr. Biden’s candidacy, as he seeks to recover momentum after a largely limp summer on the campaign trail. He has slipped in the polls over the last two months and faces an increasingly formidable challenge from Senator Elizabeth Warren: Two polls released in recent days showed her slightly overtaking him in Iowa and New Hampshire, the leadoff states in the Democratic nomination process.
In the short term, at least, Mr. Trump may have effectively put Hunter Biden off limits for his father’s Democratic rivals. It is unlikely that any other candidate in Mr. Biden’s party would soon follow Mr. Trump in going after the former vice president’s son on the subject of business ethics, while Mr. Trump may face an impeachment inquiry for allegedly pressuring a foreign government to do just that.
Senator Bernie Sanders, for one, deflected a reporter’s question on Tuesday about whether he thought the sudden focus on Hunter Biden’s overseas work would weaken the Biden campaign. “You guys write very perceptive articles on these things, and I’ll let you make that judgment,” he replied.
It is unclear, however, how well prepared Mr. Biden and his campaign may be to address questions about Hunter Biden’s business dealings beyond the specific case Mr. Trump has attempted to raise. Hunter Biden has operated for years in parts of the business world that intersected with his father’s career in government, including working for MBNA, a Delaware-based bank, while his father was involved in rewriting the bankruptcy code as a senator, and leading an investment fund that did business in foreign countries while his father was vice president.
Hunter Biden’s work for MBNA briefly became an issue during his father’s campaign for the vice presidency in 2008, and Obama-Biden campaign officials defended it as within the bounds of ethical propriety.
TJ Ducklo, a spokesman for Mr. Biden, said the former vice president was indeed angry that Mr. Trump was “lying about his family, and now appears to be blackmailing a foreign government into helping legitimize those lies.”
“He’s fighting back, which is what Democrats want to see in their nominee and why he will beat Donald Trump next fall,” Mr. Ducklo said, adding, “The V.P. and his family know Donald Trump plays dirty, and they can take it.”
Mr. Ducklo declined to say whether the campaign had vetted Hunter Biden’s business dealings and said the media should not be asking questions like that. “We’re not going to indulge Trump’s conspiracy theories, and we’re not going to dignify his attacks with a response,” he said.
Mr. Biden is said to have limited patience for discussing Hunter Biden’s apparent political vulnerabilities, even in private. Some of Mr. Biden’s close advisers have argued in private settings that going after any member of Mr. Biden’s family — including Hunter Biden — would ultimately backfire on an opponent, because of Mr. Biden’s stature and the sympathetic feelings much of the public has for him and his family.
For Mr. Biden, the matter of his son is deeply sensitive for reasons that have nothing to do with politics: While the whole Biden family endorsed his choice to enter the 2020 race, friends of Mr. Biden said consistently throughout his deliberations that his deepest anxiety was the impact of a campaign on a family still grappling with profound loss after the death of Mr. Biden’s elder son, Beau Biden, in 2015, from brain cancer.
“I know that he worked through that, he did a lot of contemplation about how it would affect the family,” said the Rev. Joseph Darby, an influential South Carolina-based pastor who has known Mr. Biden for years.
Earlier this year, Hunter Biden spoke in detail to The New Yorker magazine about the crippling grief he experienced after his brother’s death, and his long-running struggles with drug addiction. The magazine story also detailed a number of potential ethical conflicts in Hunter Biden’s business activities and described the vice president as having dealt “with Hunter’s activities by largely ignoring them.”
Maggie Astor and Sydney Ember contributed reporting.