Johnson, a Wisconsin Republican, similarly said during another radio interview this week that the evidence his committee had uncovered was so "outrageous" that "it should completely disqualify Biden from president."

The Biden campaign called the comments explicit proof of what Democrats have been claiming all along: that Johnson's probe of corruption allegations against the intelligence community and Biden's diplomatic efforts in Ukraine were thinly veiled efforts to weaponize the powerful Senate Homeland Security and Government Affairs Committee to damage the presumptive Democratic presidential nominee.

"This damning acknowledgment totally exposes that Ron Johnson's disgraceful conduct is the definition of malfeasance," said Biden spokesman Andrew Bates. "It is beyond time for him to end this embarrassing and deeply unethical charade once and for all — as a number of his Senate Republican colleagues have long wanted."

As Election Day approaches, Johnson has found himself besieged by the left and the right, distrusted by some intelligence officials and facing allegations that his committee has partly relied on information obtained from a Ukrainian lawmaker whom the U.S. intelligence community has now deemed a tool of a Russian election interference effort. (Johnson says he hasn't received anything from the lawmaker, Andrii Derkach). Johnson, who claims he's being targeted for destruction by Democrats and the press, also hinted in one Tuesday radio interview that he had some friction with Senate Majority Leader Mitch McConnell, who he said had "sidelined" him at one point during his investigation.

Asked about Johnson's comment, McConnell aides said it would be up to Johnson to elaborate. A source close to Johnson said McConnell's decision to tap the Senate Intelligence Committee to investigate Russian interference in the 2016 election — combined with the lengthy investigation by special counsel Robert Mueller — "made obtaining documents and information very difficult." The source noted that McConnell had voiced general support for aiming subpoenas at former Obama administration officials.

In short, Johnson increasingly finds himself on an island while presiding over a politically loaded investigation less than 100 days before the election. The contours of his investigation are a bit blurry, overlapping with a similar probe into alleged intelligence community abuses by Senate Judiciary Committee Chairman Lindsey Graham, even as Johnson vows to ramp it up and issue a full report on his findings in September.

Johnson’s probe examines allegations of corruption within the U.S. intelligence community during the transition of power from the Obama administration to President Donald Trump, as well as claims of abuses by intelligence community officials stemming from the FBI's investigation of Russian interference in the 2016 election, known as Crossfire Hurricane.

Johnson has emphasized that his investigation's overlap with Graham is another reason he hasn't pursued certain lines of inquiry. Graham is pursuing allegations of abuses by the FBI in its investigation into the Trump campaign's 2016 contacts with Russia. That overlap became particularly evident this week: Johnson subpoenaed the FBI on Monday, demanding all records related to Crossfire Hurricane and accusing Director Chris Wray of stonewalling his investigation.

Yet on Thursday, Graham issued a statement insisting that Wray "is committed to being helpful — in an appropriate manner — by balancing the needs of privacy for Bureau employees with public transparency for the benefit of the American people." Graham made no mention of Johnson's subpoena and noted that Wray had vowed to share information with his committee.

Trump has repeatedly encouraged investigations into former President Barack Obama, claiming without evidence that Obama committed grave crimes against Trump’s incoming administration. Trump dubbed the alleged scandal “Obamagate” but has offered no details to support allegations that Obama committed any wrongdoing.

Johnson is also pursuing widely discredited allegations that Biden engineered the removal of a Ukrainian prosecutor to shield his son Hunter from a corruption probe. At the time, Hunter was serving on the board of Burisma, a Ukrainian energy company, a conflict that several Obama-era officials said presented the appearance of a conflict of interest even if they saw no evidence of wrongdoing.

A series of State Department officials told Congress during impeachment proceedings against Trump that Ukraine's top prosecutor at the time, Viktor Shokin, was an impediment to anti-corruption efforts and Biden's push to remove him was part of the U.S. government and international community's efforts to root out bad actors in Ukraine. Shokin's ouster made it more likely — not less — that Burisma would face a serious investigation, witnesses said.

The latest indications of pent-up anger on the right that Johnson’s probe hasn’t gone far enough came during a contentious radio interview Wednesday with the usually friendly conservative host Hugh Hewitt, who told Johnson he had "failed" in his investigation by declining to subpoena key Obama-era figures like FBI Director James Comey and CIA Director John Brennan. During an occasionally heated 10-minute exchange, Johnson attributed his pace to resistance from multiple Republicans on his committee, who he said could block him from issuing subpoenas.

But Johnson's office later acknowledged this wasn't the case — the committee's Republicans already voted to empower Johnson to subpoena Brennan, Comey and others during a June business meeting. Rather, aides said Johnson had opted against issuing subpoenas because he wanted to exhaust efforts to obtain documents and seek voluntary cooperation from witnesses.

Aides to the Wisconsin Republican declined to discuss the status of those negotiations, but a source familiar with the probe indicated that Brennan has not been contacted by the committee about the prospect of voluntary testimony. The source close to Johnson, however, indicated that the panel is in talks with a "dozen or so" witnesses related to the Ukraine inquiry and that interviews are being scheduled.

"The committee is going through the process of building documents and a schedule for additional interviews in an organized manner," the source said.

Johnson has faced pushback from within his own party. Sen. Mitt Romney of Utah has criticized the probe as appearing overtly political, though he later relented and approved Johnson's subpoena authority, saying he was given an assurance that witness interviews would be done behind closed doors to avoid a political spectacle.

Johnson took aim at his critics earlier this week with an 11-page letter, accusing unnamed Democrats and media outlets of trying to topple his probe with allegations of Russian disinformation, while being guilty of disseminating it themselves.

"The very transparent goal of their own disinformation campaign and feigned concern is to attack our character in order to marginalize the eventual findings of our investigation," Johnson wrote. "They are running the same play, out the same playbook they have been using for the last three and a half years."

Asked about the attacks on his probe during a Tuesday radio interview with a conservative host in Wisconsin, Johnson put a finer point on it: "There's a coordinated effort now to destroy me."