Sen. Ron Johnson, a top Republican ally of President Donald Trump, said a U.S. diplomat told him in August that the Trump administration had frozen almost $400 million in aid to Ukraine because the president wanted a commitment that Kyiv would carry out investigations related to U.S. elections.

The Wisconsin senator, in an interview with The Wall Street Journal published Friday, said he called Trump about the allegation on Aug. 31, and that Trump vehemently denied it.

"He said—expletive deleted—'No way. I would never do that. Who told you that?" Johnson recounted Trump as telling him in the call, according to the paper.

Johnson told The Wall Street Journal that he was informed of the allegations by U.S. ambassador to the European Union Gordon Sondland.

Speaking to reporters in Sheboygan, Wisconsin, before The Wall Street Journal story was published, Johnson said that during the Aug. 31 talk, he asked Trump to "give me the authority to tell [Ukrainian] President [Volodymyr] Zelinskiy that we were going to provide that" aid.

"I didn't succeed," Johnson said, according to audio of the interview posted by the Milwaukee Journal Sentinel.

As for why not, Johnson said Trump had concerns, including about "what happened in 2016."

"He was very consistent on why he was considering it. It was corruption overall generalized, but yeah, no doubt about it, what happened in 2016, what happened in 2016, what was the truth about that, and then the fact that our NATO partners don’t step up to the plate,” Johnson said.

In a statement provided to NBC News, Johnson's office stressed that the term "quid pro quo" was not used.

"Senator Johnson does not recall in any meeting or discussion with the president, or any member of the administration, that the term 'quid pro quo' was ever used. Nor does he recall any discussion of any specific case of corruption in the 2016 election, such as Crowdstrike, the hack of the DNC servers, Hillary Clinton campaign involvement, or Hunter and Joe Biden, during general discussions of corruption, which is endemic throughout Ukraine," the statement said.

Johnson told The Wall Street Journal that he’d reached out to Sondland after discovering the aid, which Johnson had advocated for, had been frozen. Sondland said the administration had been working on an arrangement with Ukraine in which the country would appoint a new prosecutor general and move to “get to the bottom of what happened in 2016—if President Trump has that confidence, then he’ll release the military spending,” Johnson told the paper.

The senator said the suggestion made him "wince." "My reaction was: 'Oh, God. I don’t want to see those two things combined,'" Johnson told the paper.

The money was released in September after news of the freeze became public and led to bipartisan push from Congress.

Johnson told reporters last month that he’d asked Trump to release the aid for Ukraine, but did not mention his conversation with Sondland.

"I spoke to the president the Friday before we left about the funding in Ukraine, to try and convince him to give me the go ahead to tell President Zelinskiy that they’ll get that funding," Johnson said. "The primary rationale he gave me is again, that same idea that Europe is not stepping up to the plate to spend the money they should spend in their own backyard. Russian interference, Russian aggression to Ukraine, certainly threatens Europe more than it does the U.S. And he just thinks we are suckers and it irritates him."

The White House on Sept. 25 released a detail summary of Trump's July conversation with Zelinskiy which shows the president asked his Ukrainian counterpart to look into possible 2020 rival Joe Biden and his son, Hunter Biden.

At another point in the call, Trump appears to ask Zelenskiy whether someone in the Ukraine might possess a server that contained some of former presidential candidate Hillary Clinton’s emails. Though it was not explicit, Trump also appears to tie the award of aid to Ukraine to Zelenskiy's willingness to cooperating with Trump.

Johnson has defended Trump's call, saying he has "no concerns."

"He wants to get the truth," Johnson told the Journal Sentinel.

Sondland’s name appears repeatedly in the whistleblower complaint about Trump that is now at the center of a formal impeachment inquiry launched by House Democrats. The complaint, made public by the House Intelligence Committee a day after the White House released the call summary, said White House officials were so concerned about the president's conversation with Zelenskiy that they intervened to "lock down" the transcript of the conversation.

The whistleblower, whose name and gender has not been released, lodged the formal complaint out of a belief that Trump was "using the power of his office to solicit interference from a foreign country" in the 2020 election.

The complaint painted Sondland, a hotel mogul with no prior diplomatic experience who donated $1 million to Trump’s inauguration, as pushing back against linking the aid to investigation. Text messages between Sondland and two other diplomats that were given to Congress, however, show him in a less flattering light.

The messages, released Thursday by House Democrats, revealed the Trump administration's pressure on Ukraine to conduct investigations into the president's political opponents.

In two instances, a career diplomat named Bill Taylor complains about Ukraine’s security aid being tied to “investigations.” Both times, Sondland told him to stop communicating by text. “Call me,” Sondland told him after the first exchange on Sept. 1.

Sondland is one of several State Department officials that the House committees leading the impeachment inquiry have requested to depose.