(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.

"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."