One of President Trump’s top supporters in the Senate is pressing ahead with a politically fraught investigation into former vice president Joe Biden and Ukraine, despite warnings from Democrats that he risks laundering Russian disinformation into the United States through the Senate ahead of the 2020 presidential election.

Sen. Ron Johnson (R-Wis.), chairman of the Senate Committee on Homeland Security and Governmental Affairs, is scrutinizing a Ukrainian gas company’s hiring of Biden’s son onto its board and the activities of a lobbying firm it hired in Washington, reviving a thread of the Trump impeachment proceedings that the president’s allies hope could damage Biden.

Johnson has said he is investigating whether Hunter Biden’s involvement with the gas company posed a conflict of interest to then-Vice President Biden’s work on Ukraine policy. His committee has been collecting documents and in recent days interviewed three witnesses, including a top State Department official who testified during the impeachment hearings, according to people familiar with the inquiry, who spoke on the condition of anonymity because of the ongoing investigation. Johnson told the Hill newspaper that he is planning to publish his report by mid-September, weeks before the Nov. 3 election.

Johnson, who is working with Senate Finance Committee Chairman Charles E. Grassley (R-
Iowa), has said Democratic objections are a sign that his inquiry is on the right track.

“My question is: What are Democrats afraid of in my investigation?” Johnson said in an interview Sunday with Milwaukee ABC affiliate WISN.

Biden’s supporters accuse the Wisconsin lawmaker of using Senate powers to give Trump the kind of investigation into Biden that the president tried and failed to coerce out of Ukraine’s leadership, resulting in his impeachment late last year. They say he shouldn’t be taking information from Ukraine.

“Senator Johnson is diverting his committee from oversight of the failing response to the pandemic — even though over 4.5 million Americans have been infected — and is instead facilitating a foreign influence operation to undermine our democracy,” Biden campaign spokesman Andrew Bates said.

Johnson’s probe is proceeding as Ukrainians with a variety of competing agendas and links to Trump attorney Rudolph W. Giuliani have been releasing apparently pilfered official conversations that Biden conducted while vice president with Ukraine’s then-President Petro Poroshenko, aiming to tarnish the presumptive Democratic nominee and his longtime interlocutor in Kyiv, as The Washington Post has previously reported.

Critics say the efforts of the Ukrainians align with the interests of certain Ukrainian oligarchs and Russia, which for years has sown doubt about cooperation between Washington and Kyiv in an effort to eject the United States from the region.

At least one of those Ukrainians, former diplomat Andrii Telizhenko, who recently released transcripts of the Biden conversations in Kyiv from an unknown source, told The Post he has been giving information to Johnson’s staff. And another Ukrainian releasing tapes, former prosecutor Kostiantyn Kulyk, last year made contact with the staff of Rep. Devin Nunes (Calif.), the top Republican on the House Intelligence Committee, according to documents released during the impeachment probe and interviews with Lev Parnas, Giuliani’s former associate.

Kulyk did not respond to requests for comment. In the past, he has denied having contact with any U.S. officials.

Leading the charge in Kyiv is Andrii Derkach, a Ukrainian lawmaker previously affiliated with a Russian-leaning party. The son of a former KGB officer who later ran Ukraine’s intelligence service, Derkach was educated at the Higher School of the KGB in Moscow and met Giuliani at least twice since late last year before releasing the Biden tapes at joint news conferences with Kulyk.

In a statement to The Post earlier this year, Derkach said questions about whether he has ties to foreign intelligence services are smears aimed at stopping his anti-corruption work.

“The purpose of my activity is pursuing the interests of Ukraine, exposing international corruption, maintaining partnership relations between strategic partners — Ukraine and the USA,” Derkach said.

He has dubbed his Biden campaign “DemoCorruption,” complete with an English-language website and YouTube channel and a conspiratorial flowchart featuring the former vice president and financier George Soros at the center. Among other things, the chart lists influential Americans who Derkach indicated are pursuing his agenda in the United States — including Johnson and Grassley.

Derkach told The Post this month that he has sent documents to Johnson’s and Grassley’s Senate committees.

“I want to point out that we are not interested in Biden as a US presidential candidate,” he wrote in a text message, adding that he was focused on his activities as vice president.

In a letter Tuesday to Democratic leaders, Johnson and Grassley said they had received information from one foreign national — Telizhenko. Spokesmen for the GOP senators said their staffs had not received any information from Derkach. The spokesmen also denied receiving any information from Ukrainian tycoon Oleksandr ­Onyshchenko, who earlier this year told The Post he had handed over materials regarding Biden.

Johnson, in the interview with WISN, said his staff hadn’t received any of the audiotapes released in Ukraine and accused the Democrats of pushing a false narrative against him.

Asked whether he was receiving information from “pro-Russia Ukrainians,” he said his staff was receiving information from a ­“variety of sources” but primarily from the U.S. government.

“Before we ever use it, we verify and make sure it’s accurate and true, before we’d ever publish anything,” Johnson said. “I would ask Democrats: What have I published, what have I reported on, that is not true, that is any form of Russian disinformation? There has been nothing.”

The situation prompted House Speaker Nancy Pelosi (D-Calif.) and Senate Minority Leader Charles E. Schumer (D-N.Y.) in mid-July to request an FBI “defensive counterintelligence briefing” to all members of the House.

“Congress appears to be the target of a concerted foreign influence campaign, which seeks to launder and amplify disinformation in order to influence congressional activity, public debate, and the presidential election in November,” Pelosi and Schumer wrote in a July 13 letter to the FBI.

The letter included a classified addendum, suggesting there was further information that the Democrats could not release publicly.

Johnson and Grassley implied in their letter to Pelosi and Schumer on Tuesday that the addendum included Derkach’s flowchart. The Republican senators suggested the contents were leaked to the media, writing: “It is you, not us, who have participated in the spread of disinformation.”

After a briefing to members of the House on Friday by U.S. intelligence officials, Pelosi said the Trump administration was refusing to share critical information about the matter with the public.

“Leader Schumer and I wrote to them and said tell the truth to the American people, and for some reason, they are withholding it,” Pelosi said. “That’s what I’m concerned about.”

Other Republican senators, including Richard Burr (R-N.C.) and Lindsey O. Graham (R-S.C.), have also expressed concerns about Senate staffers taking what could be Russian disinformation sent through Ukrainians and using it as the basis for a probe.

Johnson declined to be interviewed by The Post. But in a statement, he said the classified addendum Pelosi and Schumer sent didn’t contain anything new of significance but had been used falsely to smear him and his investigation.

“I will not be deterred by these despicable tactics designed to discredit a legitimate investigation,” Johnson said. “It only increases my curiosity: What do they know that we might uncover?”

In his interview with WISN, the senator suggested his inquiry was looking at whether Hunter Biden’s involvement with the Ukrainian gas company, Burisma, represented a conflict of interest for the vice president and influenced U.S. policy toward Ukraine — a narrower focus than the letters he has sent the administration requesting information suggested.

So far, the Homeland Security and Finance panels have received documents from the State Department, the National Archives, the Treasury Department and Blue Star, a Democratic-led lobbying firm hired by Burisma to help settle its legal troubles, people familiar with the probe said.

Bipartisan staff members from Johnson’s and Grassley’s panels have interviewed three witnesses — Deputy Assistant Secretary of State for European and Eurasian Affairs George Kent, who testified during the impeachment inquiry; David Wade, who was chief of staff to then-Secretary of State John F. Kerry; and Foreign Service officer Liz Zentos, deputy political counselor at the U.S. Embassy in Afghanistan, who served as National Security Council director for Eastern Europe during the Obama administration.

The decision to interview Zentos appears to stem from an interview Telizhenko gave to journalist John Solomon, who was working closely with Giuliani last year, in which he alleged that Burisma was discussed during a White House meeting with Ukrainian officials in January 2016 at which Zentos was present.

Joshua A. Levy, a lawyer for Zentos, said she told congressional staffers that “in all of the government meetings in which she participated, she has no recollection of Burisma or Hunter Biden coming up in any of them.”

“That includes the January 2016 meeting with Ukrainian and U.S. officials she recalls attending,” Levy said in a statement.

The panels intend to continue through the August recess interviewing current and former State Department officials who worked on Ukraine policy with Biden, the people familiar with the probe said.

In his interview with the Hill, Johnson suggested that he may soon subpoena Antony Blinken, a top Biden foreign policy adviser who served as deputy secretary of state during the Obama administration, and Amos Hochstein, a former top State Department official who advised Biden on energy matters in Ukraine.

Johnson would need Democrats’ permission or an affirmative vote from the committee to do so because neither Blinken nor Hochstein is on the list of current and former officials for whom the panel has preapproved summonses.

A longtime watcher of Ukraine issues, Johnson was the chairman of the Senate Homeland Security and Governmental Affairs Committee during the final two years of the Obama administration, but he didn’t investigate Hunter Biden’s involvement with Burisma then, even though reports in U.S. newspapers raised questions about it at the time.

Johnson’s investigation began last year as counterprogramming to the House impeachment process. During those proceedings, Democrats showed how Trump withheld a sought-after White House meeting and U.S. military aid from Ukraine’s newly elected leader while pressuring him to announce an investigation into the Bidens.

Sen. Ron Wyden (Ore.), the top Democrat on the Finance Committee, said nothing he had seen in the current probe showed that U.S. policy toward Ukraine, which Johnson supported, changed as a result of Hunter Biden’s involvement with Burisma. Wyden said the investigation “started as an effort to distract from Donald Trump’s impeachment and has morphed into an effort to boost Donald Trump’s campaign.”

Johnson’s inquiry appears to be pursuing an allegation Giuliani has made for months but failed to prove: that Joe Biden pushed for the firing of Ukraine’s prosecutor general in late 2015 and early 2016 to help Burisma, whose owner, a former government minister from the now-
defunct Russian-leaning Party of Regions, was under scrutiny by Ukrainian authorities.

In fact, at the time, Biden was the lead voice in a coalition of U.S. and European officials who were demanding the prosecutor general’s ouster for failing to go after high-level corruption cases and stalling reforms. Biden has said he never discussed the company with his son.

In a 2016 letter, one of the U.S. officials demanding reform in the Ukrainian prosecutor general’s office was Johnson.

Bates, the Biden campaign spokesman, said Johnson is “engaged in an act of stunning hypocrisy,” given that he endorsed Biden’s activities at the time and for years after.

In a statement last year, Johnson said his change in tune was not hypocritical, suggesting he may have been subject to a “misinformation campaign against the Ukrainian prosecutor general, perpetrated by representatives of the U.S. government.”

As part of their inquiry, Johnson and Grassley requested that the National Archives provide records of White House meetings with officials from the Ukrainian government and the Democratic National Committee, appearing to follow up on unsubstantiated accusations, also promoted by Giuliani, that the Democratic National Committee colluded with Ukraine to interfere in the 2016 election on Hillary Clinton’s behalf.

For years, the main proponent of that theory has been Telizhenko.

Telizhenko originally worked at the Ukrainian prosecutor general’s office, then at the Ukrainian Embassy in Washington, and later for Blue Star. More recently, he has been coordinating with Giuliani, organizing the former mayor’s trip to Kyiv in late 2019, and promoting Trump.

In the past, the former diplomat has told The Post that he has been working as a consultant with various Ukrainian tycoons and political figures, not always on a paid basis, but more recently he said he had temporarily dropped his clients.

Originally, Telizhenko appeared on track to serve as a key witness in Johnson’s inquiry, but Sen. Mitt Romney (R-Utah) withheld support for subpoenaing him.

Earlier this year, the committee staff received a classified briefing regarding Telizhenko that raised significant questions, according to U.S. officials, who spoke on the condition of anonymity to describe the confidential session.

Telizhenko said that Democrats on the commitee “tried to politicize some information that they had on file.” He said he has been providing information voluntarily.

In an interview with The Post earlier this year in Kyiv, Telizhenko said he has been keeping in touch with U.S. officials and people close to Congress and the White House, whom he declined to identify, as well as Giuliani.

At the same time, back in Kyiv, Telizhenko also has been publishing transcripts of private taped conversations Biden had with Poroshenko.

Telizhenko originally told The Post he received tapes of the calls from a former top Ukrainian official close to Poroshenko, whom he declined to name, but later said he obtained the tapes from “a journalistic source connected to the government.”

In the WISN interview Sunday, Johnson said the committee was going to subpoena records only from when Telizhenko worked for Blue Star, the U.S. lobbying firm that Burisma hired. “How could that be Russian disinformation?” Johnson asked.

But earlier this year, Telizhenko told The Post he had forwarded more than 100 old emails to Johnson’s staff, including messages from his time at the Ukrainian Embassy to and from members of the White House National Security Council. He said he also had been answering questions posed by Johnson’s staff.

Telizhenko has said he isn’t operating in conjunction with Derkach, the Ukrainian lawmaker who began leaking some of the same tapes at the same time. Derkach has said he received the tapes from “investigative journalists.”

In recent months, Derkach has been publishing portions of the tapes — which are unverified and appear to be heavily edited — during joint news conferences with Kulyk, the former Ukrainian prosecutor who was introduced to Nunes’s staff by Parnas last year.

Derkach, who visited the United States as recently as February to meet with Giuilani, said in March that the State Department revoked his U.S. visa, suggesting the U.S. government has been keeping tabs on his work.

Sen. Chris Murphy (D-Conn.) asked Secretary of State Mike Pompeo at a July 30 hearing whether Derkach should be viewed as a credible source of information and, if the U.S. government possesses information suggesting otherwise, whether it will be released.

“When it’s appropriate, I will,” Pompeo said. “When there’s still work ongoing, when there is still unsettled intelligence around these things, I’m going to try to be a little bit more careful.”

In an interview with The Post, Murphy said that the intelligence community knows a lot about what Russia is trying to do to interfere in the 2020 election, and “there’s no reason for that information not to be made public before the election.”

If the administration has information connecting Derkach’s actions to Russia, , Murphy said, “then it is nearly treasonous for them not to disclose that fact to the American public before the election.”

Correction: A previous version of this story referred imprecisely to how a flowchart touted by Ukrainian lawmaker Andrii Derkach describes his relationship with GOP Sens. Ron Johnson and Charles E. Grassley. The story has been updated to make clear that the flowchart reflects Derkach’s view that they are pursuing his agenda.

Stern reported from Kyiv. Ellen Nakashima, Tom Hamburger and Carol Morello in Washington contributed to this report.