Democratic presidential candidate Joe Biden answers a question as President Trump listens during the presidential debate at Belmont University in Nashville, Tenn., on Thursday. (Pool/Reuters)

Joe Biden leaned heavily on a letter from former U.S. intelligence and defense officials in Thursday night’s debate to argue that Russia orchestrated a disinformation operation allegedly involving damaging information obtained from his son’s laptop that was promulgated by President Trump’s personal attorney, Rudolph W. Giuliani.

“There are 50 former national intelligence folks who said that what he’s accusing me of is a Russian plant,” Biden said at the debate. The former vice president said those former officials had concluded that what Trump was citing about his son was “a bunch of garbage.”

But the former intelligence and defense officials who penned the letter explicitly said they had no evidence of Russian involvement, noting only that Giuliani had been the target of Russian spies and their experience made them “deeply suspicious” that the Russian government played a role.

The Biden campaign’s decision to lean into accusations of Russian involvement in the episode, despite lacking specific proof, risks eroding public trust in U.S. allegations of foreign election interference if the suspicions in this case turn out to be unfounded, according to intelligence and foreign policy experts. Trump already has undermined such trust by casting doubt on proved Russian interference on his behalf during the 2016 campaign and denigrating U.S. intelligence officials.

“Biden is holding himself to a higher standard than Trump. Then we should hold Biden to a higher standard as well,” said Thomas Rid, professor of strategic studies at Johns Hopkins University and author of a book on the history of disinformation and political warfare. “And that means acknowledging in this case that we just don’t have the evidence.”

The Biden campaign’s assertion of Russian involvement in the Hunter Biden leaks comes as the Democratic nominee campaigns on restoring truth and transparency to the U.S. government.

Trump’s director of national intelligence, John Ratcliffe, said publicly that there is no intelligence to support allegations that the leak of materials related to Hunter Biden was “part of a Russian disinformation campaign.”

But Ratcliffe’s statement was interpreted broadly as a partisan gesture designed to bolster a disinformation campaign being launched directly by Trump’s allies, rather than a formal assessment from the U.S. intelligence community.

There are also indications that the Trump administration has sought to downplay the threat of Russian interference in the campaign. A senior Department of Homeland Security official said in September that he was told to stop providing intelligence reports on the Russian threat, in part because it “made the President look bad.”

“The problem of the situation we are currently in is that we have an absence of credible individuals within the intelligence community,” said Susan Hennessey, a senior fellow at the Brookings Institution and former attorney for the National Security Agency. “There is no one who can say, in a credible manner, either there is intelligence to support this claim that this is Russian disinformation or there is no information to support that claim.”

The Biden campaign has defended the former vice president’s accusation of Russian involvement in the episode by pointing to circumstantial evidence and Russia’s track record of such behavior, including in this election cycle.

“It has long been indisputable that Russia is actively interfering in our election to denigrate Vice President Biden and help President Trump,” said Andrew Bates, a spokesman for the Biden campaign. He said the letter states “the most recent smears bear all the hallmarks of a Russian disinformation campaign.” Bates added: “We know who is behind this, and it is the same hostile foreign power whose assistance Donald Trump has repeatedly courted while giving them impunity for placing bounties on the heads of American service members.”

Giuliani claims he retrieved the materials he gave to the New York Post from liquid-damaged laptops Hunter Biden had dropped off at a Delaware computer shop in April of 2019 and failed to retrieve.

U.S. intelligence officials warned the White House late last year that Giuliani was the target of a Russian influence operation. The former New York mayor and confidante of the president has met multiple times with a Ukrainian lawmaker whom the U.S. government sanctioned in September for being an “active Russian agent for more than a decade.”

That lawmaker, Andrii Derkach, had been undertaking what the U.S. Treasury described as a foreign influence operation; he had been leaking tapes of Biden conducting diplomacy with Ukraine’s leadership to impugn the Democratic nominee’s integrity ahead of the Nov. 3 vote. Derkach, who attended the Dzerzhinsky Higher School of the KGB, has denied acting as a foreign agent for Russia.

In August, National Counterintelligence and Security Center Director William Evanina said in a statement: “We assess that Russia is using a range of measures to primarily denigrate former Vice President Biden and what it sees as an anti-Russia ‘establishment.’”

In an interview, Lev Parnas, who functioned as Giuliani’s fixer in Ukraine last year, also said he attended a lunch at which Giuliani was told Ukrainians and Russians possessed compromising materials regarding Hunter Biden. Parnas said Giuliani expressed interest in the material, but Parnas didn’t see it, doesn’t know if it existed and doesn’t know if Giuliani ever obtained it. Parnas’s claim was first reported by Politico.

When asked whether he would accept dirt on his opponents from foreigners last year, Trump said he would take it, and that has bolstered doubts about the provenance of the material his lawyer has released on the eve of the election.

“Donald Trump can thank himself for why people won’t take this seriously,” said Joe Trippi, a Democratic strategist who is not working on Biden’s campaign. Trippi said the Biden’s campaign tactic, to remain largely silent, makes political sense. “It’s crazy to do anything to fuel the insanity.”

The Biden campaign has, with a few exceptions, refused to engage on the substance of the leaked material, declining to deny or corroborate alleged emails excerpted in the New York Post, because they believe that would elevate the disinformation operation.

Hunter Biden’s lawyer has also declined to say whether his client dropped off laptops with the Delaware computer repairman whom Giuliani said he retrieved the materials from. The computer repairman, for his part, has declined to say how he connected with Giuliani.

The Washington Post has not been able to independently verify the emails. The Post has on multiple occasions asked Giuliani and Trump’s former top adviser, Stephen K. Bannon, for copies of what they allege is Hunter Biden’s hard drive but has received no response.

During a heated exchange in Thursday’s debate, Biden, in his first extended remarks on the topic, said he has been told the episode is a “Russian plan.”

“You mean the laptop is now another Russia, Russia, Russia hoax?” Trump replied.

“That’s exactly what — that is exactly we've been told,” Biden replied.

Biden’s campaign said later that the former vice president was referring to the letter signed by 50 former members of the intelligence community that said the episode “has all the classic earmarks of a Russian information operation.”

Five former directors or acting directors of the CIA signed the letter, including Michael Hayden, who led the agency under presidents George W. Bush and Barack Obama, and Leon Panetta, who was both CIA director and secretary of defense during the Obama administration.

The former intelligence officials said a number of factors led them to conclude there was Kremlin involvement. They said such a “laptop” operation would be consistent with Russian objectives, methods and tradecraft, and they also pointed out that Giuliani was the target of a Russian influence operations and had obtained materials from Derkach. A Ukrainian gas firm that placed Hunter Biden on its board, they added, was allegedly hacked by Russian intelligence.

Still, the former officials cautioned: “We do not have evidence of Russian involvement.”

Hennessey said given that he is not currently the president, Biden is free to come to whatever conclusions he wants based on the publicly available evidence. But she urged caution generally when attributing a disinformation operation to a foreign power.

“When we think about it from the perspective of restoring U.S. faith in the findings of the U.S. intelligence community and informing the public — from that perspective, we want to be extraordinarily cautious, never get ahead of the evidence and be restrained,” she said.

Rosalind S. Helderman contributed to this report.