US 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 whistleblower 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-neighbor 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 matchups.

Impeachment probe

The impeachment investigation 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 seeking a suspension of all efforts surrounding the impeachment inquiry 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.

The White House plans to send Pelosi a letter, possibly on Friday, that will argue Trump and his team can ignore lawmakers’ demands until she holds a full House vote formally approving an impeachment inquiry, a source familiar with the plan said.

Axios was first to report the plan.

Pelosi 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 $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.

On Thursday, the Wall Street Journal said 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 the Bidens.

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

A longtime US diplomat who served as Trump’s special envoy for Ukraine spoke to congressional committee members and staff as part of the Democratic-led impeachment probe.

Kurt Volker resigned as special representative for Ukraine negotiations on Friday, the day after the public release of the whistleblower complaint that described him as trying to “contain the damage” from efforts by Giuliani to press Ukraine to investigate Democrats.

A person familiar with the briefing said Volker and Gordon Sondland, the US ambassador to the European Union, had drafted a statement for Zelenskiy that would have committed Ukraine to investigate the Bidens and unsubstantiated allegations that Ukraine, not Russia, had meddled in the 2016 US presidential election on behalf of Hillary Clinton.

The drafting of the statement was first reported by the New York Times.

Giuliani told Reuters on Thursday that Volker never expressed any concerns about his efforts and put him in touch with Andriy Yermak, an adviser to Zelenskiy.

“There are texts where he asked me to report back to him on Yermak,” Giuliani said. “There are texts that show that I did. There’s never a text anywhere that says he’s concerned about anything. And he never said that to me.”

The Ukraine investigation has angered Trump, whose volatile presidency survived a two-year federal investigation 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, retweeted on Thursday 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 U.S. 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 $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,” Pence told reporters in Arizona.”And the president has made it very clear that he believes [...] other nations around the world should look into it as well.”

Biden’s deputy campaign manager Kate Bedingfield said in a statement that Trump’s comments on Thursday showed he was “desperately clutching for conspiracy theories that have been debunked”.

The House Intelligence Committee’s chairman, Democrat Adam Schiff, accused Trump of “a fundamental breach of a president’s oath of office” because of his China comment.