President Trump’s re-election campaign has run menacing and misleading ads this fall accusing Joseph R. Biden Jr. of corrupt dealings with Ukraine. Republicans in Congress are scrutinizing Mr. Biden’s son, pressing the State and Treasury Departments for information about his work for a Ukrainian energy company. The president himself has unleashed a stream of unfounded accusations against the Bidens and pushed for them to appear at a potential impeachment trial in the Senate.
As Mr. Trump faces impeachment for allegedly pressuring Ukraine to investigate Mr. Biden, he and his allies are now turning those same claims about Mr. Biden and his son into a key element of their defense. And they plan to continue to hammer at the Bidens’ Ukraine dealings as impeachment proceedings move into the new year.
There is no evidence that the elder Mr. Biden, while serving as vice president, improperly intervened in Ukraine to benefit his son. But the president’s advisers are betting that many voters will ignore the complexities of the allegations and absorb a simple message about a father using his influence to help his son, a senior Trump campaign official said.
The barrage of attacks is the latest example of Mr. Trump’s monthslong fixation with Mr. Biden, who leads the Democratic field in national polling and has bested the president in head-to-head surveys in several key battleground states. Mr. Biden and many independent experts have called the allegations of corruption false, and Mr. Biden’s campaign insists that they demonstrate how worried the president is about facing him in a general election.
But the prospect of a public airing of the White House’s corruption claims lasting into the primary season worries some Democrats, who see a parallel to Republican efforts to tarnish Hillary Clinton over her email practices in the run-up to the 2016 election. That campaign, they say, proved how damaging a constant stream of insinuations and accusations from Mr. Trump and his allies can be.
“The tactic is very similar,” said Karen Finney, a Democratic consultant and former Clinton campaign senior spokeswoman. “Republicans were doing all they could to amp up the questions about her emails. They didn’t necessarily argue the facts. All they had to do was throw in the question to raise doubts.’’
Some voters at Mr. Biden’s campaign stops in recent weeks were keenly aware of that history.
Kristin Tracy, 50, who attended an event with Mr. Biden in Des Moines, said that “there must be something to Joe that’s scaring the president, the current president, or he wouldn’t be trying to drum up those investigations against him with the Ukrainians.”
Still, she allowed, the scrutiny of Mr. Biden made her nervous.
“It obviously worked against Hillary Clinton,” she said of Mr. Trump’s attacks. “There’s a lot of people that don’t know, believe in blind faith what’s said on Fox or out of the president’s mouth.”
Hunter Biden did hold a lucrative position on the board of the Ukrainian energy company Burisma while his father was vice president, and while there is no evidence of wrongdoing, the arrangement struck some Obama administration officials as unseemly given the elder Mr. Biden’s role in Ukraine policy.
Now, some Democrats worry that a continued spotlight on Mr. Biden’s family will expose the former vice president’s vulnerabilities against a president who relishes getting personal. On Thursday in Iowa, Mr. Biden engaged in an angry exchange with a voter who falsely accused Mr. Biden of having “sent” his son to work in Ukraine, and of “selling access to the president.”
“You’re a damn liar, man,” Mr. Biden said.
The moment showed Mr. Biden being forceful, at a time when some Democrats want him to more vigorously defend himself. But it was also an emotional and heated reaction to a predictable issue that Mr. Trump would be sure to press in a general election.
The Trump campaign is divided over whether Mr. Biden will emerge as the nominee, the senior Trump campaign official said. Some remain convinced that Senator Elizabeth Warren will emerge as the standard-bearer. But others have noted that Mr. Biden hasn’t collapsed despite months of scrutiny and a series of missteps, and see the entry of Michael R. Bloomberg, a billionaire who has already spent at least $50 million on television advertising since getting in the race less than two weeks ago, as representing the kind of chaos that will hurt lower-polling candidates, and not Mr. Biden.
The campaign believes its message about Ukrainian corruption will help them even if Mr. Biden is not the nominee — that it can be used to associate Democrats with a status quo that Mr. Trump ran against in 2016, and is hoping to again.
Still, many Democrats say that Mr. Biden is better equipped to respond to Mr. Trump and congressional Republicans this cycle than the Democrats were in 2016.
Mr. Trump is on the defensive himself, facing damaging revelations from the impeachment proceedings. Ms. Finney and other veterans of Mrs. Clinton’s campaign also noted that the news media routinely describes many Republican claims about the Bidens as baseless, a difference from the tone of the coverage about Mrs. Clinton’s emails in 2016 that has, so far, been helpful for Mr. Biden.
And Mr. Biden, a longtime Delaware senator who had extensive bipartisan relationships on Capitol Hill, simply does not have the same baggage with congressional Republicans that Mrs. Clinton did dating to her time as first lady, some Democrats say.
“Joe is not Hillary, and Hunter is not the candidate,” said former Senator Bill Nelson, a Florida Democrat who is supporting Mr. Biden. He suggested the difference in dynamics between Mr. Biden and Mrs. Clinton was also partly due to sexism.
Republicans plan to keep pressing for Mr. Biden to be called to testify, both to highlight the issue of Hunter Biden’s consulting work and to suggest that Mr. Biden is being protected by Democrats in Washington.
Asked whether he would appear voluntarily at a trial, Mr. Biden told reporters Wednesday: “No, I’m not going to let them take their eye off the ball. The president is the one who has committed impeachable crimes, and I’m not going to let him divert from that.”
The president’s allies, in the meantime, have made clear they intend to make the Bidens the story. Senator Lindsey Graham’s request to the State Department seeks to uncover new information about the Bidens’ dealings in Ukraine. Senators Chuck Grassley of Iowa and Ron Johnson of Wisconsin have pressed for documents from the Treasury Department seeking evidence of money laundering or fraud related to Hunter Biden’s business payments.
Spokesmen for Senators Graham, Grassley and Johnson did not grant interview requests.
“The Bidens are in the middle of this,” said David Winston, a veteran G.O.P. pollster who works with congressional Republicans. “No matter which side you’re looking at it from.”
Mr. Biden, his advisers and his allies are making the case that Mr. Trump is attacking the former vice president because he fears him as a potential opponent. The very fact that Mr. Trump pressed the president of Ukraine to investigate the Bidens proves that point, they say.
“I might say this is the only thing that Donald Trump and I agree on is the one he’s most concerned about,” said former Gov. Tom Vilsack, an Iowa Democrat who traveled with Mr. Biden on a weekend swing across the state.
Mr. Biden has turned the issue into one of his most reliable applause lines on the campaign trail — “Donald Trump doesn’t want me to be the nominee”— and his campaign is eager for Mr. Biden to use Mr. Trump as a foil at every turn.
Mr. Biden’s campaign also argues that the Republican concerns about the Bidens’ records in Ukraine didn’t surface until politically convenient.
“Republicans controlled both chambers for years after all of this became public and didn’t say one word about it until Trump was exposed trying to blackmail Ukraine into bailing out his re-election campaign,” said Andrew Bates, a spokesman for the campaign.
Those dynamics weren’t enough to reassure Tom McGrane, 69, as he waited to see Mr. Biden at a campaign event in rural Denison, Iowa, last weekend. He said the Republican approach to Mr. Biden reminded him of another historical parallel: the 2004 presidential election, when the organization Swift Boat Veterans for Truth attacked John Kerry’s military record.
“As long as Republicans keep hammering on it, I think he needs to stop thinking it’s just all going to go away,” said Mr. McGrane, who said Mr. Biden should spend more time denouncing the Republicans’ actions against him.
Mr. McGrane told Mr. Biden just that after a speech here.
“He said he understands it completely, ” Mr. McGrane recalled afterward. Mr. Kerry endorsed Mr. Biden on Thursday.
Speaking to reporters recently, Mr. Biden briefly appeared to acknowledge the risk at hand, as he referenced a television interview Hunter Biden did earlier this fall.
“He said, although he didn’t do anything wrong, he wishes he hadn’t had bad judgment because it allowed folks like Trump to try to change the focus,” Mr. Biden said.
He soon pivoted, saying, “This is about Donald Trump and his corrupt behavior.”
Thomas Kaplan contributed reporting from Cedar Rapids, Iowa.