CEDAR RAPIDS, Iowa — Since President Trump defeated them nearly three years ago, Democrats have warned that Mr. Trump would once again benefit from the interference of foreign governments to help bolster his re-election bid.
Now, as reports that he sought help from the Ukrainian government shake the political world, former Vice President Joseph R. Biden Jr., the monthslong leader in the primary race, finds himself grappling with the fallout of a still-secret whistle-blower complaint that is said to be about Mr. Trump and his dealings with Ukraine.
For Mr. Biden, it is both the contrast he wants and the controversy he would rather avoid.
The revelations offered voters a preview of what is likely to be an extraordinary general election contest if Mr. Biden were to win the nomination, one in which attacks by the president and his team could boomerang, transforming Mr. Biden into a sympathetic figure under ugly attack with foreign help.
It could just as easily mark a defining moment for Mr. Biden, a 76-year-old politician first elected to the Senate in 1972 and long accustomed to playing by the more genteel political rules of a different era.
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While the new report gives Mr. Biden the one-on-one showdown with Mr. Trump that his campaign has spent months trying to create, it also exposes him and his son, Hunter Biden, to yet another round of probing questions about their moneymaking activities and personal family struggles.
At a time when some of the candidates had been shifting their strategy from trying to chip away at Mr. Biden’s persistent lead to attacking the ascendant candidacy of Senator Elizabeth Warren of Massachusetts, the new reporting once again places Mr. Biden at the center of the 2020 campaign.
Mr. Biden, whose appearances on the campaign trail can be halting and sprinkled with misstatements, has generally delivered his strongest performances when focused on Mr. Trump. Speaking about the president allows Mr. Biden to discuss foreign policy and national security, issues that his campaign has said differentiate Mr. Biden, a former Senate Foreign Relations Committee chairman, from the rest of the 2020 Democratic field.
Yet Mr. Biden’s initial response was to brush off the new revelations and stick to his campaign schedule in Iowa, offering only a meager retort.
“I have no comment except the president should start to be president,” he told reporters, walking quickly to a waiting van.
At issue are demands from Mr. Trump and his personal lawyer, Rudolph W. Giuliani, that Ukraine examine Mr. Biden’s dealings with the country when he was vice president at the same time that his younger son, Hunter Biden, was doing business there.
Mr. Trump and Mr. Giuliani have reportedly pressed for an investigation of the Bidens for weeks, after reports this year in The New York Times and elsewhere examined whether a Ukrainian energy company had sought to buy influence in Washington by hiring Hunter Biden. The younger Biden had a lobbying business in Ukraine while his father was vice president.
Mr. Biden’s team believes the accusations that his son improperly leveraged his family name on behalf of his lobbying clients have already been widely debunked in the media. Still, the re-emergence of the younger Biden’s business dealings this week invites a new round of scrutiny from the press, allies of Mr. Trump and Mr. Biden’s many remaining opponents in the primary.
On Saturday morning, Mr. Trump posted a video mash-up of TV news footage of stories about Mr. Biden’s son. “This is the real and only story,” the president wrote.
So far, Mr. Biden’s rivals, nearly all of whom descended on Iowa this weekend, have resisted taking the bait. Several of his competitors were quick to assail Mr. Trump on Friday, while avoiding commentary about how Mr. Trump’s accusations against the Bidens would affect the Democratic nominating contest.
Gov. Steve Bullock of Montana said Mr. Trump had exhibited the “behavior you’d expect from an incompetent mobster.” Representative Tim Ryan of Ohio urged Congress to hold hearings and investigate the whistle-blower’s allegations. And Senator Cory Booker of New Jersey, whose campaign manager on Friday released a memo stating he would have to drop out of the race if he failed to raise $1.7 million before the end of September, remarked that “this is not a partisan issue,” while Mr. Booker and former Representative Beto O’Rourke of Texas reiterated their calls for Mr. Trump to be impeached.
“I’m going to keep the focus on the fact that Donald Trump has broken the law if this report is accurate and he should be impeached,” former Housing Secretary Julián Castro said during an interview Friday night in Cedar Rapids. “That’s where the focus belongs right now.”
Ms. Warren, who first called for Mr. Trump to be impeached in April after the release of a report by the special counsel Robert S. Mueller III, also renewed those demands, but went even further, arguing that by failing to act on impeachment in preceding months, Congress had become “complicit in Mr. Trump’s latest attempt to solicit foreign interference to aid him in US elections.”
“Today’s news confirmed he thinks he’s above the law,” she said. “If we do nothing, he’ll be right.”
Even if Mr. Biden’s primary competitors don’t take direct aim, the perception of the Biden family leveraging its connections — even if little more than a conspiracy theory — cuts a stark contrast with his two leading rivals, Senators Elizabeth Warren and Bernie Sanders, who have centered their candidacies around a fierce populist message of rooting out corruption in Washington.
It’s a message that worked in 2016 for Mr. Trump, who cast Hillary Clinton as the avatar of establishment self-dealing, a past-her-prime creature of Washington unable to adjust to the times and produce real change.
Mr. Biden’s team is acutely aware of that comparison, and a few hours after Mr. Biden’s non-comment, his campaign decided to go further.
Sensing an opportunity to highlight Mr. Trump’s fixation with Mr. Biden, his aides released a statement in his name blistering the president for “abhorrent” conduct and demanding Mr. Trump release the transcript of his call with the Ukrainian leader and allow the director of national intelligence to release the whistle-blower’s claims to Congress.
“There is only one candidate the president is trying to get foreign governments to dig up bogus dirt on,” said Anita Dunn, a senior adviser to Mr. Biden.
But Mr. Biden stopped short of offering a full-throated endorsement of moving forward with impeachment proceedings, an idea that has been gaining traction on Capitol Hill and within the Democratic primary field despite opposition from House Speaker Nancy Pelosi.
Advisers to Mr. Biden said his initial reluctance reflected his prudence about discussing sensitive national security matters rather than unease with the work of his son in Ukraine. But the former vice president is highly sensitive about questions regarding his family, and it was not until other outlets had confirmed the initial Wall Street Journal report that the Biden campaign determined it should try to go on the offensive.
His appearances on Friday were marked by moments of testiness.
While taking questions outside a Cedar Rapids nature center Friday afternoon, Mr. Biden offered a sharp retort to Angie Weiland, a 59-year-old dental hygienist, who pressed him to support a single-payer health care plan like one backed by Mr. Sanders and Ms. Warren.
“God love you, you’ve got the right candidate in Bernie or Elizabeth or whoever you have,” Mr. Biden said. “Tell Elizabeth it’s going to cost a lot of money and you have to raise people’s taxes.”
A few hours later, Mr. Biden snapped at the female moderator leading a forum on L.G.B.T.Q. issues after she questioned his decades-old votes for laws outlawing same-sex marriage and prohibiting gays and lesbians from serving openly in the military, as well as his contention, offered in February and then quickly retracted, that Vice President Mike Pence is “a decent guy.”
Before an audience of 700 activists at Coe College in Cedar Rapids, Mr. Biden sarcastically called his inquisitor, the Cedar Rapids Gazette columnist Liz Lenz, “a lovely person,” prompting her to reply: “Just asking the questions people want to know.”
Offstage after their exchange, she wrote on Twitter that Mr. Biden had called her “a real sweetheart,” a comment she said in a subsequent interview that she found to be “a little condescending.”
By Friday night, Mr. Biden’s campaign was fully embracing the argument that Mr. Trump’s attempted intervention with Kiev was evidence about which candidate he did not want to run against, blasting out a fund-raising email that even alluded to Hunter Biden. “Donald Trump asked a foreign leader eight times to investigate my family,” the money request went. “But I’m only going to ask you once.”
While Mr. Biden may embrace that message, his rivals have repeatedly questioned his age and his grasp on the fiercely polarized politics of the Trump era.
Senator Amy Klobuchar of Minnesota cited her past three years in the Senate as experience that differentiated her from Mr. Biden.
“I’ve been able to navigate through this recent era, the Trump era,” she said. “I’ve been living it for two years.
Matt Stevens and Katie Glueck contributed reporting from New York and Jonathan Martin contributed reporting from Washington.