President Donald Trump on Thursday again invited foreign interference in a US presidential election by publicly calling on China to investigate Democratic political rival Joe Biden, the kind of request that has already triggered an impeachment inquiry in Congress.

The Republican president said he believed both China and Ukraine should look into 2020 presidential hopeful Biden and his businessman son Hunter, and described the impeachment probe as”crap.”

Trump told reporters at the White House, “And by the way, likewise, China should start an investigation into the Bidens. Because what happened in China is just about as bad as what happened with Ukraine.”

Trump and his personal lawyer, Rudy Giuliani, have offered no evidence for their assertions of corruption regarding former Vice President Biden, a leading contender for the Democratic Party nomination to run against Trump.

Questioned about whether he had asked Chinese President Xi Jinping to investigate, Trump said, “I haven’t but it’s certainly something we can start thinking about.”

The Chinese embassy in Washington did not immediately respond to a request for comment.

Trump’s appeal to China was particularly striking, since Washington and Beijing are locked in a bitter trade war that has damaged global economic growth. They are set to hold another round of talks in the United States next week.

The China remarks stiffened the resolve of Democrats in the House of Representatives to push ahead with an inquiry into whether Trump should be impeached following revelations by an anonymous whistle blower who said Trump asked Ukraine in July to investigate the Bidens.

Hunter Biden was on the board of a Ukrainian gas company at a time when the United States and Europe were trying to woo Ukraine away from next-door-neighbour Russia.

Some Republicans have criticised Biden for taking his son on the Air Force Two vice presidential plane in 2013 to China,where Hunter had business interests.

At a healthcare event in Florida, Trump said Democrats had launched the impeachment investigation because they could not beat him at the ballot box in 2020.

“That’s why they do the impeachment crap – they know they can’t win,” Trump said.

While the election is still a long way off, Reuters/Ipsos polling shows Trump narrowly trailing the top Democratic candidates, including Biden, in hypothetical match ups.