Johnson, during an interview with WIBA, a Wisconsin radio station, brought up Trump's comments and argued that Democrats should support an investigation before the party picks its 2020 nominee. 
"I mean where is the curiosity of the press?" Johnson continued. 
Trump, speaking to reporters outside of the White House, urged both China and Ukraine to open investigations into the Bidens. 

"China should start an investigation into the Bidens,” Trump said.

He added that he had not explicitly asked Chinese President Xi Jinping to open an investigation, but that it’s “certainly something we can start thinking about.”
Republicans have largely been silent about the president's latest remarks as they scattered across the country for a two-week break.
Trump's suggestion to Ukrainian President Volodymyr Zelensky that he investigate the Bidens, and a related whistleblower complaint, has fueled the House Democrats impeachment inquiry. 
Johnson in a separate interview with reporters in Wisconsin said that Trump is the country's "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,” he said.