Sen. Ron Johnson Ronald (Ron) Harold JohnsonThe Hill's Morning Report - Sponsored by The Air Line Pilots Association - White House moves closer to Pelosi on virus relief bill Second GOP senator to quarantine after exposure to coronavirus GOP-led panel to hear from former official who said Burisma was not a factor in US policy MORE (R-Wis.), a staunch ally of President Trump Donald John TrumpUS reimposes UN sanctions on Iran amid increasing tensions Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Trump supporters chant 'Fill that seat' at North Carolina rally MORE, on Tuesday urged former national security adviser John Bolton John BoltonDiplomacy with China is good for America The Hill's Morning Report - Sponsored by The Air Line Pilots Association - Pence lauds Harris as 'experienced debater'; Trump, Biden diverge over debate prep DOJ launches probe into Bolton book for possible classified information disclosures MORE to speak out publicly about what he knows about efforts to hold up military assistance to Ukraine.

Johnson, the chairman of the Senate Homeland Security and Governmental Affairs Committee and member of the Senate Ukraine Caucus, invited Bolton in early January to testify before the Senate, but Bolton informed the lawmaker he would only respond to a Senate subpoena.

“I spoke with John Bolton on Jan. 7 when I heard that he wanted to testify,” Johnson said, recalling his conversation with Bolton in connection with his own committee’s investigation into possible corruption in Ukraine.

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“I said 'John, if you’ve got something to say, I’d rather have you say it sooner rather than later. We’re calling in a bunch of witnesses, why don’t you come into our committee?' John at that time said, ‘I’d only respond to a Senate subpoena,’ ” he said.

Johnson is one of three GOP chairmen, along with Senate Judiciary Committee Chairman Lindsey Graham Lindsey Olin GrahamMomentum growing among Republicans for Supreme Court vote before Election Day Video of Lindsey Graham arguing against nominating a Supreme Court justice in an election year goes viral Warning signs flash for Lindsey Graham in South Carolina MORE (R-S.C.) and Finance Committee Chairman Chuck Grassley Charles (Chuck) Ernest GrassleySenate Republicans face tough decision on replacing Ginsburg What Senate Republicans have said about election-year Supreme Court vacancies Biden says Ginsburg successor should be picked by candidate who wins on Nov. 3 MORE (R-Iowa), who are probing conflicts of interest in the Obama administration when Hunter Biden was serving as a highly-paid board member of Burisma Holdings, a Ukrainian energy company.

In the wake of a New York Times report that Bolton claims in an unpublished manuscript that Trump explicitly linked military assistance for Ukraine to an investigation of former Vice President Joe Biden Joe BidenMomentum growing among Republicans for Supreme Court vote before Election Day Trump expects to nominate woman to replace Ginsburg next week Video of Lindsey Graham arguing against nominating a Supreme Court justice in an election year goes viral MORE and his son, Johnson now says Bolton should go public with what he knows.

Johnson said Bolton should clarify what he knows but suggested he do so outside the formal proceedings of the impeachment trial.

“Now that what has unfolded with the manuscript being leaked — by the way, exquisite timing, maybe suspicious timing — The Wall Street Journal has called for John to just come forward. Just tell the public what you know. I think that would actually be a smart thing. I’d encourage John to do that.”

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Johnson said Bolton should do so “without involving the trial” and possibly go straight to the media.

The Wall Street Journal’s editorial board opined Monday that “the former NSC adviser should tell the public what he knows.”

“With the news of what’s in the book already public, Mr. Bolton can help everyone, including himself, by erasing any doubt about what he knows. He can tell the American public what he wrote—now, before the Senate votes on witnesses. Lay it all out. Put to rest the 'coverup' talking point,” the paper wrote.

Johnson, however, did not give any indication that he would vote for a motion to subpoena additional documents or witnesses and it’s considered unlikely that Bolton will discuss his conversations with the president outside of the impeachment trial.