Wisconsin Senator Ron Johnson may end up being a key witness in the ongoing impeachment proceedings against President Trump after he spoke with the President about aid to Ukraine ahead of a visit to the country in early September.

"I'm the one that called President Trump a few days before that [visit] and really tried to convince him to release the funding," Johnson told "The Dan O'Donnell Show." "I heard his explanation for why he was concerned [about releasing it]. He has been incredibly consistent really from the day he's entered office: He's upset that Europe doesn't step up to the plate, that [German Chancellor] Angela Merkel doesn't commit more funds to help Ukraine and NATO since it's in their backyard and not ours, and he obviously was concerned about corruption."

President Trump reportedly decided in late August to withhold $400 million in aid to Ukraine because of these concerns. Democrats in the House of Representatives launched an impeachment inquiry into his phone call with Ukrainian President Volodymyr Zelensky a month earlier in which Trump discusses joint efforts by the two countries to combat corruption in Ukraine under Zelensky's predecessor.

"Yes, you are absolutely right," Zelensky said when Trump brought up Germany failing to live up to its obligations to Ukraine. "Not only 100%, but actually 1000% and I can tell you the following; I did talk to Angela Merkel and I did meet with her I also met and talked with Macron and I told them that they are not doing quite as much as they need to be doing on the issues with the sanctions. They are not enforcing the sanctions. They are not working as much as they should work for Ukraine."

President Trump then asked Zelensky to "do us a favor" and "find out what happened with this whole situation with Ukraine, they say Crowdstrike... I guess you have one of your wealthy people... The server, they say Ukraine has it. There are a lot of things that went on, the whole situation. I think you're surrounding yourself with some of the same people. I would like to have the Attorney General call you or your people and I would like you to get to the bottom of it."

After Zelensky agreed, Trump referenced former Vice President Joe Biden's dealings in Ukraine.

"There's a lot of talk about Biden's son, that Biden stopped the prosecution and a lot of people want to find out about that so whatever you can do with the Attorney General would be great," Trump said. "Biden went around bragging that he stopped the prosecution so if you can look into it... It sounds horrible to me."

The crux of the impeachment inquiry is an investigation into whether Trump was withholding U.S. aid until Zelensky authorized an investigation into Biden, a Democratic presidential candidate who may end up opposing Trump in the 2020 election.

"I had a meeting with Zelensky with Senator [Chris] Murphy," Johnson said. "[Zelensky] was obviously concerned about the funding but in no way, shape, or form expressed any concern that any pressure was being put on him."

Murphy, a Democrat, agreed with this assessment in an interview on CBS' "Face the Nation" Sunday.

"In my meeting, to be clear, Zelensky did not, you know, make any connection between the aid that had been cut off and the requests that he was getting from [President Trump's attorney Rudy] Giuliani, he said.

Dan O'Donnell connects the dots and explains why this may end up exonerating President Trump in a lengthy segment on Monday's show. Click on the player below to listen!