MADISON, Wis. — Republican Sen. Ron Johnson, of Wisconsin, said Thursday that he doesn't think there is anything wrong with China and the Ukraine assisting U.S. authorities in investigating former Democratic Vice President Joe Biden and his son, as President Donald Trump has called for those countries to do.

Trump has called for investigations into Biden, a leading Democrat seeking to run against him next year, despite no evidence of any wrongdoing. Trump on Thursday publicly called on China to investigate Biden and affirmed the approach he made in private to Ukraine's president that has led to the impeachment investigation by Democrats in the House.

Johnson told reporters after an event in Madison that although he doesn't think those investigations are improper, he thinks American intelligence agencies would do a better job.

"The president of the United States is our chief law enforcement officer," Johnson said. "We have cooperative agreements with countries to investigate potential crimes, so I don't think there's anything improper about doing that. 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 in America."

Hunter Biden served on the board of a Ukrainian gas company at the same time his father was leading the Obama administration's diplomatic dealings with Kyiv. Although the timing raised concerns among anti-corruption advocates, there has been no evidence of wrongdoing by either the former vice president or his son.

The president and his personal lawyer, Rudy Giuliani, also have tried to raise suspicions about Hunter Biden's business dealings in China, leaning heavily on the writings of conservative author Peter Schweizer. On Monday, Geng Shuang, a spokesman for China's Ministry of Foreign Affairs, called the allegation that Chinese government business gave Biden's son $1.5 billion "totally groundless."

Johnson, chairman of the Senate Homeland Security Committee, said he thinks it was appropriate for American law enforcement to enlist the aid of other countries, even though he said, "I don't trust China any further than I can throw them at this point."

"There are plenty of unanswered questions about what happened with Hunter Biden and his group in terms of attracting capital from China," Johnson said of the younger Biden.

Johnson said the American people and Democratic presidential primary voters should know whether the Bidens did anything illegal, he said.

"You don't think they'd want to know that before they start casting their ballots in a Democratic primary?" Johnson said. "Does that make sense, do you want that coming out during the campaign? ... If something happened in 2016, I want to know, the American people ought to know."

When speaking to the Middleton Chamber of Commerce, Johnson downplayed concerns about Trump's phone call with the president of the Ukraine that sparked the impeachment probe. Trump has defended his contact with Ukraine as "perfect."

"I look at that transcript and it's Trump being Trump," Johnson said of the call. He said Democrats and the media were putting "the worst possible construction" on the call.

"I want to know if something happened in the Ukraine, I want to know if nothing happened in the Ukraine, so we can move on," Johnson said.

Trump has sought to implicate Biden and his son in the kind of corruption that has long plagued Ukraine. Hunter Biden served on the board of a Ukrainian gas company, Burisma, at the same time his father was leading the Obama administration's diplomatic dealings with Kyiv.

Democrats are talking about of basing an impeachment charge of obstruction on the White House's slow-walking of documents and testimony _ administration actions that echo the months of resisting Congress in its other investigations into special counsel Robert Mueller's report and Trump's business dealings.