SHEBOYGAN - U.S. Sen. Ron Johnson was blocked by President Donald Trump in August from telling Ukraine's president that U.S. aid was on its way amid accusations Trump was withholding it until the eastern European nation investigated his political rival.

Trump rejected Johnson's request after also refusing in May to back new Ukrainian President Volodymyr Zelensky, the Oshkosh Republican said Friday.

“I was surprised by the president’s reaction and realized we had a sales job to do,” Johnson said during a constituent stop in Sheboygan. “I tried to convince him (in August) to give me the authority to tell President Zelensky that we were going to provide that. Now, I didn’t succeed."

Johnson told reporters Trump said he was considering withholding the aid because of alleged corruption involving the 2016 U.S. election. Johnson stood by the president, saying he was sympathetic to his concerns and didn't see any bad motives on his part.

"What happened in 2016? What happened in 2016? What was the truth about that?” Johnson said about Trump's concerns.

With his comments Friday, Johnson made clear that he was aware of allegations Trump was withholding aid to Ukraine for political reasons weeks before the public knew.

Trump, who faces a fast-moving impeachment inquiry over the matter, has denied the claim and Johnson has defended the president — but Johnson's story helps House Democrats learn more about a key aspect of the probe.

Johnson, the chairman of the Senate Homeland Security Committee, was part of a delegation that attended Zelensky’s inauguration in May. He and others briefed Trump on the inauguration and urged Trump to back Zelensky, but the president resisted the idea, Johnson said Friday.

“We all went in there having come in from the inauguration and, you know, we were trying to encourage the president to show a great deal of support,” Johnson said. “Oval Office visit, appoint an ambassador who could be appointed quickly on a bipartisan basis — because we came back from meeting President Zelensky pretty confident, pretty encouraged that he really does understand what his mandate is and he’s dedicated to fulfilling it."

Trump was not receptive to the message from the delegation, which included Johnson; Gordon Sondland, the U.S. ambassador to the European Union; and Kurt Volker, the State Department special envoy to Ukraine at the time. Volker stepped down last week and testified behind closed doors Thursday as part of the impeachment inquiry.

Johnson told the Wall Street Journal on Friday that Sondland told him that Ukraine would appoint a prosecutor who would, as Johnson put it, work to "get to the bottom of what happened in 2016 — if President Trump has that confidence, then he’ll release the military spending."

“At that suggestion, I winced,” Johnson told the Wall Street Journal. “My reaction was: Oh, God. I don’t want to see those two things combined.”

He said he asked Trump about it and the president denied it.

"He said — expletive deleted — ‘No way. I would never do that. Who told you that?'” Johnson told the Wall Street Journal.

In an interview this week with the Milwaukee Journal Sentinel, Johnson said the call lasted 10 to 15 minutes and Trump said he was holding the money back because of corruption concerns.

“He was very consistent in why he hadn’t made that decision (to release the aid) yet,” Johnson said. “He said, ‘Ron, do you know how fricking corrupt that place is?’”

Trump said he hadn’t made a final decision, but he thought Johnson would like it when he reached it, Johnson said.

When Johnson met with Zelensky days later, the Ukranian president asked about the U.S. aid, Johnson said.

“At no point in time did Zelensky ever mention or indicate that he was feeling pressure,” he said. “He was just concerned, he said, and by the way — far more important than the funding is just to show support.”

Johnson said he told Zelensky not to worry about the funding because there was unanimous support for it in Congress and lawmakers would make sure his country got it.

Democratic Sen. Chris Murphy of Connecticut was also at the meeting. Johnson said Murphy told Zelensky not to work with Rudy Giuliani, Trump’s personal attorney who has sought to dig up dirt on Biden in Ukraine. Murphy has given a similar account of his comments to Zelensky.

Johnson on Friday also sought to walk back comments he made Thursday about whether the president acted properly by calling on a foreign government to investigate Joe Biden, the former vice president who is running for president.

Johnson's call with Trump over the allegation suggests he may not be comfortable with the president using the power of his office to ask foreign countries to investigate a potential 2020 rival.

That's in contrast to comments Johnson made Thursday to reporters in Wisconsin.

"I don’t think there’s anything improper about doing that," Johnson said Thursday when asked whether he agreed with the president asking Chinese officials to investigate Biden and his son, Hunter.

But by Friday, Johnson said he didn't think it was appropriate for the president to use the power of his office to push a foreign country to do so.

"No, and I'm not sure that's what's happening," he told reporters in Sheboygan.

Johnson and his aides said Johnson meant there's nothing wrong with a president asking foreign governments to provide information for U.S. investigations.

"He did not comment on what the president is reported to have said," spokesman Patrick McIlheran said Friday, despite Johnson making specific references to Biden's son's business dealings in China in his answers to questions about Trump's request.

Trump said Thursday that China "should start an investigation into the Bidens" as he faces impeachment over a similar request of the president of Ukraine and just months after Trump's 2016 campaign was investigated over its ties to Russian officials.

Democrats criticized Trump over the request, saying the president was inviting another foreign power to interfere in the next presidential election just as Russia did in 2016.

Bill Glauber of the Journal Sentinel contributed to this report from Pewaukee.

Eric Litke reported from Sheboygan. Molly Beck and Patrick Marley reported from Madison.

Contact Molly Beck and Patrick Marley at molly.beck@jrn.com and patrick.marley@jrn.com. Follow both on Twitter at @MollyBeck and @patrickdmarley.