MIDDLETON - U.S. Sen. Ron Johnson said Thursday there's nothing improper about President Donald Trump's call on Chinese officials to investigate his top political rival in his 2020 re-election bid.

Trump extended the invitation Thursday to the foreign country 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 Russia officials.

Johnson told reporters Thursday it's appropriate for Trump to seek help from other nations to determine whether criminal acts have been committed.

"I want to find out what happened during 2016," he said after visiting a meeting of the Middleton Chamber of Commerce. "If there's potential criminal activity, the president of the United States is our chief law enforcement officer. We have proper agreements with countries to investigate potential crimes so I don't think there's anything improper about doing that."

It was unclear Thursday what potential crimes Trump believes the Bidens may have committed in China.

Trump has contended that Hunter Biden persuaded China to contribute $1.5 billion to an investment fund around the time he traveled to the country with his father aboard Air Force Two in 2013, a claim an attorney for Hunter Biden has denied, according to the Washington Post.

Johnson said he doesn't trust China "any farther than I can throw them" and that U.S. agencies would conduct a better investigation, but he said U.S. intelligence officials also could use their information.

"I don't know what may or may not have happened with China and the Bidens, but I think an awful lot of those investigations can actually occur here in America," Johnson said. "The American people have a right to know. Something wrong happened as it relates to politics and the 2016 election — we have a right to know that."

Johnson made stops in Waukesha and Dane counties this week as the U.S. House Democrats continue their impeachment inquiry into Trump's effort to work with foreign countries to investigate former Vice President Joe Biden, his potential rival in the 2020 election.

At an event in conservative Pewaukee on Wednesday, the audience didn't mention the inquiry, but in more-liberal Middleton — it was the topic of one of the first questions.

Johnson reiterated Thursday the Democrats' characterization of a phone call Trump made to Ukrainian President Volodymyr Zelensky misrepresents its significance.

"I look at that transcript and I go, it's Trump being Trump," Johnson said.

Johnson has called for a review of whether Biden improperly used his office as vice president to remove Ukraine's top prosecutor in order to prevent him from investigating a company Biden's son oversaw as a board member.

CNN reported Thursday Johnson signed a letter in 2016 urging then-Ukrainian President Petro Poroshenko to make "reforms" in the same office.

Johnson said in Middleton he didn't remember the letter.

"Those countries are trying to shed the legacy of corruption — endemic corruption throughout their societies," Johnson said, speaking generally. "I send out all kinds of oversight letters ... I don't know which 2016 oversight letter you're referring to so I will look at that and then we'll issue a press release, statement or something — but I don't engage in hypocrisy. I'm looking at getting the truth."

Johnson later issued a release on a more-recent letter he wrote expressing concerns about a potential "misinformation campaign" against the Ukrainian prosecutor general by the Obama administration.

Contact Molly Beck at molly.beck@jrn.com. Follow her on Twitter at @MollyBeck.