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(CNN) —  

House Republicans are asking a Republican senator who spoke to President Donald Trump and the US Ambassador to the European Union about the freezing of US security aid to Ukraine to provide information relevant to the Democrats’ impeachment inquiry.

The House Oversight Committee’s ranking Republican, Rep. Jim Jordan of Ohio, and the House Intelligence Committee’s ranking Republican, Rep. Devin Nunes of California, sent a letter to Sen. Ron Johnson of Wisconsin on Saturday asking him to provide “firsthand” information relevant to the impeachment probe.

“Because the Democrats have abandoned fundamental fairness and objectivity in their ‘impeachment inquiry,’ we reluctantly write to request any firsthand information you have about President Trump’s actions toward Ukraine between April and September 2019,” Jordan and Nunes wrote.

Johnson, who has been involved in several key moments related to Ukraine this year, said on NBC’s “Meet the Press” Sunday that he did not expect to testify before House impeachment investigators, but that he would provide a written statement.

“They’re not going to call me because certainly (House Intelligence Chairman) Adam Schiff wouldn’t want to be called by the Senate,” Johnson said. “There’s going to be a separation there, but I think I will reply to that and I will supply my telling of events which is difficult to do in eight to 10 minutes on a show like this.”

Johnson added that he believed the whistleblower complaint would have been “far better off if we were just taking care of this behind the scenes.”

“Having this all come out into public has weakened the relationship (with Ukraine), has exposed things that didn’t need to be exposed,” he said.

Johnson was part of the US delegation led by Energy Secretary Rick Perry that attended Ukrainian President Volodymyr Zelensky’s inauguration in May. Along with the “Three Amigos” — Perry, US Ambassador to the European Union Gordon Sondland and former US special envoy for Ukraine Kurt Volker — who took the lead on Ukraine policy, Johnson briefed Trump on Ukraine at a May 23 meeting.

Bill Taylor, the top US diplomat in Ukraine, testified the group supported Zelensky, but Trump expressed skepticism toward Ukraine, pointing to an unsubstantiated conspiracy theory that Ukraine interfered in the 2016 election. Trump told them they should talk to his personal attorney Rudy Giuliani about Ukraine.

In late August, Johnson spoke by phone to Sondland, and has said that the EU ambassador told him that the frozen security aid money was contingent on Ukraine announcing investigations. “At that suggestion, I winced. My reaction was: Oh, God. I don’t want to see those two things combined,” Johnson told the Wall Street Journal in October.

Johnson said he called Trump afterward, and Trump said that was not the case.

“So, I’m the one that raised the issue from my phone call with Gordon Sondland the day before, where he described some type of – something that Ukraine had to do before President Trump would release the funding,” Johnson said on CNN’s “State of the Union” earlier this month. ” And when I brought up that scenario, President Trump immediately – and I have described as adamantly and vehemently – denied it.”

CNN’s Phil Mattingly contributed to this report.