What you need to know about Ron Johnson, Donald Trump and the impeachment inquiry

U.S. Sen. Ron Johnson's name keeps coming up in the impeachment inquiry of President Donald Trump. 

Johnson attended the inauguration of Ukraine's president, met with a Ukrainian diplomat and questioned Trump this summer on whether aid was being withheld until Ukraine launched investigations. Trump denied any wrongdoing.

Here's a look at why the Republican senator from Oshkosh has become an important figure in the inquiry.

Johnson briefed Trump and why that matters

Johnson was part of the U.S. delegation that attended the May 20 inauguration of Ukraine President Volodymyr Zelensky.

Johnson is chairman of the Europe subcommittee on Senate Foreign Relations Committee, chairman of the Senate Homeland Security Committee and a member of the Senate's bipartisan Ukraine Caucus.

Kurt Volker, the U.S. envoy for Ukraine, briefed the delegation on reports that Trump's personal attorney, Rudy Giuliani, was trying to get Ukraine to conduct investigations that would help Trump politically, according to the Wall Street Journal. (Johnson has not said whether he got that briefing.)

Three days after the inauguration, on May 23, Johnson and the others talked to Trump in the Oval Office about the inauguration and encouraged him to back Zelensky. Trump reacted skeptically.

The Oval Office meeting created an 'irregular channel'

Gordon Sondland, the U.S. ambassador to the European Union, has testified Trump directed the delegation to discuss Ukraine issues with Giuliani. Johnson has said he does not recall Trump mentioning Giuliani. 

Diplomat William Taylor testified that an "irregular channel" for foreign policy with Ukraine was established at that Oval Office meeting.

Taylor testified he grew alarmed by August because the irregular, politically connected channel for foreign policy developed different objectives than the normal foreign policy channel maintained by the State Department.

Johnson met with a Ukrainian diplomat

Weeks later, Johnson's aides invited Ukrainian diplomat Andrii Telizhenko to meet with Johnson on July 11 or so, according to Telizhenko. Telizhenko has contended his bosses directed him to help Democrats in the 2016 election by gathering dirt on Trump's campaign chairman, Paul Manafort. 

Telizhenko and Johnson talked for at least 30 minutes and discussed the notion that Ukraine interfered with the 2016 election, according to The Washington Post. Telizhenko also met with Senate staff for about five hours. 

Conspiracy theory has taken root with some conservatives

Some conservatives have promoted an unsubstantiated conspiracy theory that Ukraine has Democratic National Committee servers that were hacked or otherwise interfered in the 2016 election. U.S. intelligence agencies have determined the Russian government was responsible for the DNC hacking and engaged in a wide-ranging effort to disrupt the U.S. election. 

Fact-checkers with PolitiFact and the Washington Post have discredited the idea that Ukraine has the DNC servers. 

Johnson has said he believes Russia interfered with the 2016 election but also has questioned whether Ukraine had a role in planting the idea that Trump's campaign colluded with Russians.

Johnson recently said on Fox News that "there’s all kinds of smoke about Hillary Clinton's campaign, the DNC being involved in the 2016 election.”

On NBC's "Meet the Press" this month he asked, "Who set (Trump) up? Did things spring from Ukraine?"

When Johnson heard about the allegation, he called Trump

Trump held up nearly $400 million in U.S. aid to Ukraine in July and weeks later, on July 25, asked Zelensky to do him a “favor” and investigate the 2016 election and Joe Biden, the former vice president now seeking the Democratic presidential nomination. Biden's son Hunter served on the board of a Ukrainian gas company. 

Trump's call with Zelensky triggered a whistleblower's complaint that led to the impeachment inquiry. 

Sondland, the U.S. ambassador, told Johnson on Aug. 30 that the aid would go to Ukraine once it took steps to investigate the 2016 election, according to Johnson.

Johnson "winced" at the possibility of such a connection and spoke to Trump the next day, on Aug. 31.

Trump told Johnson there was no quid pro quo

In their Aug. 31 phone call, Johnson asked Trump whether Ukraine needed to do anything to get its aid and Trump angrily insisted there was no quid pro quo, according to Johnson. Johnson took him at his word.

Also in that conversation, Johnson asked Trump to give him the authority to tell Zelensky the aid would be provided to Ukraine, but Trump wouldn’t give him that power. Trump told Johnson he would probably like the decision Trump would ultimately make on releasing the aid to Ukraine, according to Johnson.

Johnson returned to Ukraine

Days later, Johnson and Democratic Sen. Chris Murphy of Connecticut met with Zelensky in Ukraine and told him his country has bipartisan support in Congress.

Johnson has said Zelensky gave no indication he felt pressured. Murphy has said Ukranian officials were concerned about ignoring Giuliani’s requests for investigations.

The Trump administration a week later released the aid to Ukraine that had been held back since July. 

Johnson will cooperate, but not recuse

Johnson hasn't been asked to testify in the House's impeachment inquiry but has said he would cooperate if asked.

“I wouldn’t resist," he told the Milwaukee Journal Sentinel of how he would react to a request to testify. "I’m always happy to tell the truth.”

Two ethics experts told the Journal Sentinel this month that Johnson should consider recusing himself from a vote on removing Trump if the matter gets to the Senate because of Johnson's involvement in the matter.

Johnson said he would not do that because the people who elected him deserve to have representation if the issue comes to the Senate.

"I would never even consider it,” he said of recusing himself.

Contact Patrick Marley at patrick.marley@jrn.com. Follow him on Twitter at @patrickdmarley.