Sen. Ron Johnson (R.-Wis.) told the Wall Street Journal that he learned of a possible quid pro quo between the Trump administration and Ukraine's government from EU Ambassador Gordon Sondland.

The big picture: Johnson said that he pressed President Trump on the issue, which allegedly linked the distribution of $400 million in U.S. military aid with a Ukrainian investigation of former Vice President Joe Biden, in an Aug. 31 phone call. "He said ... 'No way. I would never do that. Who told you that?'" Johnson told the Journal of his conversation with the president.

Johnson asked the Justice Department earlier this week to look into links between Ukraine and Hillary Clinton's 2016 presidential campaign. His letter questioned the narrative that Biden's actions in Ukraine had nothing to do with his son's connection.

However, it was also discovered that he signed onto a 2016 letter that mirrored Biden's work to reform Ukraine's prosecutor general and judiciary.

There is no evidence that Biden's reform work in Ukraine was to protect his son's business dealings, as the president and some of his allies allege, or that he committed any form of corruption.

The state of play: Sondland, a Trump political appointee who donated $1 million to the president's inaugural committee, denied that there was a quid pro quo between the Trump administration and Ukraine's government in a series of text messages that former Ukraine special envoy Kurt Volker provided to a group of House committees investigating the Trump-Ukraine matter.

"I think it’s crazy to withhold security assistance for help with a political campaign," Bill Taylor, the top U.S. diplomat to Ukraine, texted Sondland.

Sondland texted back: "Bill, I believe you are incorrect about President Trump's intentions. The President has been crystal clear no quid pro quo's of any kind."

The New York Times reported that Sondland's response came only after he spoke to the president.

Go deeper: House committees release texts from former Ukraine envoy Kurt Volker