The agency's reluctance to engage with the Senate Homeland Security and Governmental Affairs Committee, which Johnson chairs, underscores the intelligence community’s doubts about the probe. And while it is unlikely to deter Johnson from moving forward, it could give Democrats more support as they seek to rebut Trump and GOP allegations that Biden is corrupt.

The episode began earlier this year, when Democrats raised concerns about Johnson’s investigation and the committee asked the FBI to brief senators. The FBI responded by saying the CIA should also participate, according to a person familiar with the matter — a highly unusual ask given that the Homeland Security Committee rarely, if ever, deals with the CIA.

On May 14, the Democratic committee staff sent an email to the CIA’s office of congressional affairs detailing the scope of the requested briefing, according to sources who described the email to POLITICO. Republican aides on the panel were copied on the email, which was unclassified.

The committee followed up the next day, but the CIA never responded. After publication of this story, CIA press secretary Timothy Barrett said, “Our Office of Congressional Affairs works closely with intelligence oversight committees to ensure they are informed of our activities.”

Democrats have long demanded what are known as “defensive briefings” from intelligence officials on potential efforts by the Russian government to promote disinformation and influence the 2020 presidential election. The CIA’s stance comes as Democratic congressional leaders have also been pushing senior intelligence officials to disclose more information to the public about Russia’s latest interference campaign.

But securing a briefing for the committee has been a challenge. In addition to potential concerns about Johnson’s probe, the CIA is wary of providing a briefing that could reveal sensitive sources and methods to a panel other than the Senate or House intelligence committees, which are the agency’s direct oversight bodies in Congress.

“There is a significant trust gap there,” said one current national security official. “At what point does someone turn from an ‘unwitting’ participant in that to a witting one?”

The official added that enough is known about certain bad actors that it should be “out of the question to consider their information legitimate investigative material,” referring to the Ukrainians who have sought to feed information to Johnson and other Trump allies on Capitol Hill.

Johnson has maintained that his investigation has nothing to do with the presidential election and that his committee fully vets all the information it receives.

Austin Altenburg, a spokesman for Johnson, said the criticisms are an example of “the bias that exists within the federal bureaucracy, including, unfortunately, our intelligence agencies.” He also accused Democrats of requesting briefings that are “untethered to our work.”

“What the email doesn’t show are the intelligence briefings and documents the committee has already received and the multiple conversations in which the agencies told Democrats that they had no further information relevant to our investigation,” Altenburg added.

The May email to the CIA’s congressional affairs office outlined what senators wanted to hear from intelligence officials as the Biden investigation ramped up. The committee specifically asked for information about the foreign nationals pushing allegations against the Bidens, including whether they have ties to foreign governments or foreign security services. Lawmakers also wanted to know more about efforts to interfere in the 2020 election more broadly.

Michigan Sen. Gary Peters, the top Democrat on the Homeland Security Committee, renewed his demand for such briefings in a letter to Johnson last month, imploring Johnson to pursue the issue if he continued with his probe. A spokeswoman for Peters declined to comment for this story.

Johnson’s aides, though, have blamed Democrats for the delay. Last week, Johnson sent a letter to CIA Director Gina Haspel requesting a slew of documents related to the origins of the federal investigation into Russian interference in 2016, but the letter did not mention the CIA’s apparent refusal to brief his committee.

Democrats first called for the briefings in March, when the FBI’s Foreign Influence Task Force briefed committee staffers about Andrii Telizhenko, a Ukrainian who has amplified disputed claims about coordination between Kyiv and the Democratic National Committee in 2016. Johnson was set to subpoena Telizhenko, but he scrapped plans for a committee vote on the matter after the briefing, according to sources familiar with the matter.

The day before, FBI Director Christopher Wray had briefed all senators in a classified setting about election security. POLITICO previously reported that Democratic senators pressed him about Telizhenko in the context of Russia’s efforts to sow disinformation in American politics, including about whether Telizhenko was a willing partner in the Kremlin-backed campaign.

The briefing was described as “combative” and “personal” by attendees, who said Johnson engaged directly with some of his detractors in the Senate as he sought to defend his investigation.

More recently, though, as Johnson has sought to highlight, all senators have received briefings from the intelligence community on threats to the integrity of the 2020 vote. Those briefings have touched on Telizhenko and Andrii Derkach, a Ukrainian lawmaker who has sent information about the Bidens to Trump allies on Capitol Hill, including Johnson.

Director of National Intelligence John Ratcliffe and other national security and intelligence officials briefed senators on Monday about election security and foreign influence, and they appeared to signal that they were preparing to disclose more information to the public about Russia’s intentions.

The House received a similar briefing last week, during which William Evanina, director of the National Counterintelligence and Security Center, acknowledged that Russia is again trying to boost Trump’s reelection and denigrate his Democratic opponent.

Evanina was chastised during that briefing by Speaker Nancy Pelosi, who along with other Democrats has urged the intelligence community to make public more details about Russia’s ongoing interference efforts.

Pelosi told CNN on Monday morning that it “could be unwitting on [Johnson’s] part — I don’t know what he knows. That’s why we want the intelligence community to tell the American people what they know, not jeopardizing sources and methods.”

“There is plenty they could be telling the American people,” Pelosi said, “and including the United States senators who may be associating with some of these people.”