But in a worrisome sign for Democrats anxious about reliving the party’s woes in 2016, the campaign’s counterattack is being drowned out by a pro-Trump network on social media and cable news that kicked into gear over the weekend, leveling accusations against Biden that have scant factual basis.

The skirmish illustrated how ill-equipped Democrats, busy battling one another in the fight for the nomination, are to compete with Trump’s online megaphone, which includes not just his own Twitter feed and that of the Republican National Committee’s leadership, but also a regiment of conservative talking heads and highly active Internet trolls who have closed ranks around the president.

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The flurry of online attacks against Biden carries echoes of conspiracy theories spread by Trump and his allies about Hillary Clinton’s health and claims about her foreign entanglements that helped turn voters against her. The onslaught comes at a crucial moment in the Democratic primary contest, as the former vice president attempts to showcase his ability to go toe-to-toe with Trump.

But with the Democratic National Committee attempting to be a more neutral arbiter than it was in 2016, when it was accused of aiding Clinton, the party helped monitor viral attacks but publicly remained silent.

“This is the first big test of the 2020 race: Is the press, are Democrats, are Democratic activists, going to let that happen again?” said Jennifer Palmieri, who was the Clinton campaign’s director of communications.

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She and other former Clinton aides have been watching with an apprehensive sense of deja vu as they see Trump solicit a foreign government’s help in attacking a political adversary.

“You need to be aggressive so people know what’s true and what’s not,” she said. “You can’t pick your fights. . . . It’s a terrible situation for a campaign to be in. But that’s life when you’re dealing with Donald Trump.”

Biden has been buffeted in recent weeks by a flood of claims on social media seeking to weaponize his debate misstatements and other verbal miscues. The attacks have come not just from the right, but from the left as well.

The assertion that Biden behaved corruptly when it came to Ukraine marked a significant escalation by Trump and his allies to elevate what are unsubstantiated claims into a dangerous threat to the 76-year-old’s candidacy.

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Zac Petkanas, the director of rapid response for Clinton’s 2016 campaign, said Democrats may have miscalculated four years ago in ignoring conspiracy theories that sprouted on the fringes of the Internet. They quickly went mainstream, Clinton’s campaign realized.

“The false attacks really were effective at sowing doubt, not only within a right-wing sphere, which is what we originally thought, but within our own supporters,” Petkanas said.

Hunter Biden served for nearly five years on the board of Burisma, Ukraine’s largest private gas company, whose owner came under scrutiny by Ukrainian prosecutors for possible abuse of power and unlawful enrichment.

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Hunter Biden was not accused of any wrongdoing in the investigation. As vice president, Joe Biden pressured Ukraine to fire the top prosecutor, Viktor Shokin, who Biden and other Western officials said was not sufficiently pursuing corruption cases. At the time, the investigation into Burisma was dormant, according to former Ukrainian and U.S. officials.

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Trump has suggested that Biden improperly used his influence as vice president, and the president appeared to confirm on Sunday that he raised the matter in a July phone call with newly elected Ukrainian President Volodymyr Zelensky. The Washington Post Fact Checker analyzed several of the claims from Trump and his allies in May and gave them three Pinocchios.

Trump took his attacks even further on Monday — without proof — after Biden told Trump to release the transcript of his calls with Ukraine.

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“Joe Biden and his son are corrupt. All right?” Trump said, as he was meeting with the Polish president at the United Nations. He added: “If that ever happened — if a Republican ever did what Joe Biden did, if a Republican ever said what Joe Biden said, they’d be getting the electric chair by right now.”

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While Clinton initially ignored escalating conspiracy theories, Biden’s campaign has mobilized its research team, attempting to find ways to combat false charges and return the focus to Trump. They point to polls repeatedly showing Biden defeating Trump and say he has rattled the incumbent president.

“In an outrageous and unprecedented act of corruption — and at the expense of our national security priorities — Donald Trump pressured a foreign country to smear the Democratic candidate he’s most afraid of facing next November,” said Andrew Bates, a Biden spokesman. “This kind of ‘self over country’ gutter politics is grossly beneath what we stand for as a country and will be aggressively fought against by the campaign. For the sake of our democracy, it is also incumbent on the press to finally internalize the lessons of 2016 and dispense with the uncritical repetition of wholesale lies simply because Donald Trump says them.”

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Biden, at a fundraiser in Iowa on Saturday, said the president’s behavior was troubling. “The more he thinks he’s likely to lose, the more erratic he becomes and the more things he does that are worrisome,” Biden said.

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Over the weekend, the Biden campaign and Trump released dueling videos on Twitter that underscored the social media imbalance Democrats are fighting.

Trump’s video was viewed about 2.7 million times — nearly five times more than Biden’s video. George Papadopoulos, a former Trump campaign adviser who was sentenced to prison last year for lying to the FBI, sent a tweet Friday night spreading another unfounded allegation against Biden — and it got more retweets than the video posted at the top of Biden’s official Twitter account.

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On Facebook, Trump’s supporters also mobilized, amplifying unfounded rumors about the vice president and his son. Conservative social media influencers Lynnette Hardaway and Rochelle Richardson, better known as Diamond and Silk, weighed in on Friday evening, with their post gaining thousands of shares hours before Biden’s campaign had put out anything more than a plain statement on the platform addressing Trump’s phone call.

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Top results on Facebook included material from pro-Trump news pages, some of which are operated by users outside the United States. Many of these pages enjoy more than a million followers, who are treated to a battery of videos and links to articles from websites that run hyper-partisan articles, often without bylines.

The online battles illustrate how the campaign is increasingly unfolding in an unregulated digital ecosystem, one in which Trump has eschewed traditional political norms.

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Social media companies maintain that they are not arbiters of truth and intervene only when there is evidence of coordinated inauthentic activity — a position that has frustrated Democrats. Although Democrats generally have put faith in institutions — the Justice Department, the media and Congress — to check Trump’s stretching presidential power, they see few avenues of recourse against the Trump fusillade, even as House Democrats debate whether to begin impeachment proceedings.

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“Regardless of who our nominee is, I think we’re going to face similar unconventional actions by the president, and so this is something that should concern the DNC,” Sen. Christopher A. Coons (D-Del.) said Saturday as he trailed Biden into the grounds of the Polk County Steak Fry in Des Moines. “I don’t have a concrete idea off the top of my head.”

The party “ought to take every possible responsible action,” Coons said, adding that one option might be filing with the Federal Election Commission. But that would probably be futile, he admitted, because the body lacks a quorum.

Brian Fallon, the chief spokesman for Clinton’s campaign, said arguing with Trump on merits was hardly ever successful and said he believes the best option Democrats have is putting Trump on the daily defensive with impeachment proceedings.

“It’s just a nasty mess. You heard that about emails,” Fallon said. “In the end, people thought it was a mountain being made of a molehill. But then they’d invariably blame Hillary for setting up the server in the first place. They’d blame her a little bit even though they thought Trump was being unfair.”

Biden in June issued a promise not to use false or purloined information and called on other candidates to do the same. But the party has not acted on a resolution that would bind all its candidates to similar norms, even though the pledge was endorsed earlier this year by state party chairs.

Democratic lawmakers have been quick to condemn the president, but they strained to formulate a way for the party to respond collectively to Trump’s claims. From staunch Biden allies to rivals for the Democratic nomination, there wasn’t a clear sense of how to maneuver tactically to quiet Trump’s megaphone or come up with a counternarrative.

Sen. Kamala D. Harris (D-Calif.) said the president’s alleged phone call was “just another on the list of issues” that warrant his impeachment.

When asked in an interview what other Democratic candidates could do to present a united front, she said, “I’m not going to tell other candidates what to do.”