Trump publicly asks China to investigate his political rival, Joe Biden

President Donald Trump has again invited foreign interference in a US presidential election by publicly calling on China to investigate his 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 against him as "crap".

"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 told reporters at the White House.

Trump and his personal lawyer Rudy Giuliani have offered no evidence for their assertions of corruption against former vice-president Biden (photo, above), 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, given that Washington and Beijing are locked in a bitter trade war that has damaged global economic growth. They are due 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 a probe into whether Trump should be impeached following revelations by an anonymous whistleblower who said that Trump asked Ukraine in July to investigate the Bidens.

Hunter Biden (photo) 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 The Villages, Florida, Trump said Democrats had launched the impeachment probe because they cannot 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 matchups.

Impeachment probe

The impeachment probe could lead to the approval of articles of impeachment - or formal charges - against Trump in the Democratic-controlled House. A trial on whether to remove him from office would then be held in the Republican-controlled Senate. Many Republicans, however, have shown little appetite for ousting him.

House Republican leader Kevin McCarthy wrote to Democratic House Speaker Nancy Pelosi on Thursday, asking that all efforts surrounding the impeachment inquiry be suspended until "customary rules and procedures are established".

He asked whether she planned to hold a vote of the full House authorising the inquiry and whether the president's counsel would be allowed to attend all hearings and depositions. Refusing to do these things would contradict previous impeachment procedures, McCarthy said.

Pelosi (photo) announced the impeachment investigation last week after the whistleblower lodged a complaint about Trump asking Ukrainian President Volodymyr Zelenskiy to look into investigating the Bidens.

Trump froze US$400 million in US aid to Ukraine shortly before speaking to Zelenskiy, prompting accusations from Democrats that he had misused US foreign policy for personal gain.

The Wall Street Journal reported on Thursday that Trump ordered the removal in May of the then-ambassador to Ukraine, Marie Yovanovitch, after months of complaints from allies that she was undermining him abroad and obstructing efforts to persuade Kiev to investigate Biden.

Asked if he was influential in the removal of the ambassador, Giuliani said: "I just turn my information over" to the State Department.

The Ukraine investigation has angered Trump, whose volatile presidency survived a two-year federal probe that found Russia had intervened to help him win the White House in 2016.

In an apparent warning to Trump, Ellen Weintraub, head of the watchdog Federal Election Commission, on Thursday retweeted a post she first sent in June about receiving foreign help in elections.

"Let me make something 100% clear to the American public and anyone running for public office: It is illegal for any person to solicit, accept, or receive anything of value from a foreign national in connection with a US election. This is not a novel concept," her post read.

Vice-President Mike Pence backed an investigation into Biden's dealings with Ukraine.

"My predecessor had a son who was paid US$50,000 a month to be on a Ukrainian board at the time that Vice-President Biden was leading the Obama administration's efforts in Ukraine, I think is worth looking into. And the president has made it very clear that he believes ... other nations around the world should look into it as well," Pence told reporters in Arizona.

- Reuters